I have no answer but it if helps in a pinch you might try "basic" gmail (which I prefer anyway because it's far faster). https://support.google.com/mail/answer/15049?hl=en contains links for both gmail versions under "switch views" in the article. Both links are functional for me in November 2020.

I just posted a reply but cannot see it. This forum software really doesn't make me warm and fuzzy. It says that I replied, and also says there are no replies, and doesn't show any replies.

This P.O.S. forum software must be blocking my posts that have a link. In Microsoftonian deproductive splendor it accepts the post, then it can't be seen, so I'm left to guess that it shadow bans it because it's not visible. Who knows, though, since it doesn't explain what it's doing. For Chrissake the link is to google.com, but maybe every link here on this technical forum is considered spam. Sorry that I can't help you. I've burned a good amount of time trying.

This is interesting if I try to open Gmail the web page is simply blank, nothing happens and there is no path appearing in the lower left corner. I then went to the 3 bar menu button in the upper right corner. By opening and surfing around in my photos and settings and just jumping around the menu, I went back and tried Gmail again and it loaded! But only once, then I had to perform the same exercise and it would load again.

I have Gmail set as my Home page. I went to options and removed Gmail as my Home page. I can now load Gmail by going to a favorite icon or typing in the address. As long as Gmail is NOT the home page it seems to work.

Whoops spoke to soon, it only worked once when I exited Firefox and then re opened it, I could no longer open Gmail. If I open a new tab I can open it. This is very strange!

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. You can click shield icon for more detail and possibly disable the protection. You can check the Web Console for messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

You can try these steps in case of issues with webpages:

You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

As I have said in several posts now on this same subject, Gmail Wont load on Firefox, Everything you have written will work once and only once. Staring with everything erased and a clean reset of Firefox, the instant you load Gmail it will be the last time. Then you need to start all over again. Changing settings in options or logging on to my Google account and then opining things and returning to Gmail will get it to load Gmail, just once.

I have been able to get Gmail to load every time so far by removing it as the home page. If Gmail is set as the home page when Firefox opens, it will not work. Gmail will not load the next time you start Firefox without performing one or more of many functions. The above instructions are a repeat from a similar post on this problem. These are extensive instructions that take a great deal of time and as stated before on this answer, it does not work more than once. If you close and restart Firefox it will happen again and again. Obviously a bug in the program. It would be nice if Mozilla would fix it.

Let the cookies expire when Firefox is closed to make them session cookies instead of using "Clear history when Firefox closes" to clear all cookies including cookies with an allow exception you may want to keep.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Browsing History
• do not clear the Cookies
• do not clear the Site Preferences

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

Are you possibly using "Clear history when Firefox closes" to clear the browsing history ?

Firefox 82+ comes with a "Redirect Tracking Protection" feature to purge tracking cookies for websites that haven't been visited recently automatically, so if you clear the history then there are no visits and Firefox can clear cookies.

You can disable the feature to purge tracking cookies automatically via the about:config page.

• privacy.purge_trackers.enabled = false

Another possibility is to make sure you do not use "Clear history when Firefox closes" to clear the browsing history or otherwise clear the history as this feature currently uses recent history visits.

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Since about a week or so ago late October or early November 2020, Gmail will not load in Firefox. I began having this problem about a week ago. Sometime in October or early November of 2020. Gmail will not load in Firefox. Clearing cookies and all cache, works one time. Then it starts all over. Resting also works, just once. This is a giant PITA. I have to reset or clear the cache every time I use Firefox or Gmail won't load. It works fine on all other browsers. If I want to use Firefox for Gmail, I have to reset it or clear it every time I start Firefox. I have to go back to Chrome to use my Gmail. Bob

I have had this same problem for a long time.  I used the new private window and gmail works now, hope this helps.