If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, 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

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You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.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 (Settings), click the padlock icon (Clear cookies and site data) for the domain in the selected tab.

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

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

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

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

You can check for issues with Enhanced Tracking Protection. 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.

• click the shield icon for more detail and possibly disable the protection

You can check the Web Console for relevant-looking messages like blocked content.

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

This is apparently a copy/paste from a reply you posted quite some time ago, and I don't believe the person asking the question then found it very helpful.

Did you actually try going to the web site I mentioned as an example to see if works or fails for you? That would be a VERY simple test for you to run before suggesting so many solutions.

This looks like a simple form to me if I checked this form in the Inspector. I do not have username and password for this website and haven't created one or try to use a Google account, but clicking Submit gives an error that email and password are required what confirms that the Submit button works. If a button doesn't work with such a simple form then this usually means that JavaScript is blocked. You didn't mention whether you get an error message or if indeed no reaction happens, even if you leave the form empty.

If I leave the form empty and click Submit, then it tells me the fields (user name and p/w) are required.

If I fill in my user name and p/w (which ARE valid since I can login via Chrome, as mentioned in my original post) and click Submit, nothing happens.

This type of error does NOT occur under most web sites that I use in Firefox, only a few, and it has only started happening in the last 7-10 days. I have not (knowingly) changed any settings, nor have I added any extensions.

Checking the update history, I find that FF was updated to 125.0.2 on April 23 (going from 124.0.2 on April 2) and the current 125.0.3 on April 30. This update history jibes with my recollection of when the problem started.

Did you try to disable Enhanced Tracking Protection ?

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

Turning off Enhanced Protection worked. Very annoying that Firefox doesn't tell me that's the problem when it is blocking something and give me the option right then and there to disable it for that one time so that I can proceed.