Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}acking-protection-firefox-desktop

I have to also mention that issue occurs not every time when Gmail is loaded but only about in 1/3 of cases. So to find out if issue is still there I have to restart Firefox usually about 4 times in a row. Well I just tried this:

• Deleted all my cache and cookies
• Run browser with hw acceleration off
• I use no special themes
• Run browser with extensions disabled one-by-one

After many Firefox restarts (as you can imagine :) the issue seems to not occur only when Firefox is run in safe-mode with disabled extensions. But when I try to isolate and disable functions one-by-one the issue still persists.

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

Are you using the default theme?

Are you using the default theme?

Yes. Never played with themes in Firefox.

Well I just tried this:

• Deleted all my cache and cookies
• Run browser with hw acceleration off
• I use no special themes
• Run browser with extensions disabled one-by-one

Did you restart the browser after each change?

ondrej123 said

Did you restart the browser after each change?

Yes. Actually multiple times after each change to test it properly.

I called for more help.