It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

You can try to create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Thanks for advice. I just clean-installed Firefox, having deleted the Application folder data and profiles folder, and checked for the problem without instaling any add-ons or changing any settings. The problem persists. Again the Safe mode is the only remedy.

Would you give this a try please :

3-bar menu => Options => Advanced => General tab :

Under 'Browsing' :   uncheck   'Use hardware acceleration when available'.

If that doesn't make a difference, would you take a look at this article :

Upgrade your graphics drivers to use hardware acceleration and WebGL

Indeed I forgot to disable hardware acceleration in the newly installed Firefox. When I did that, it solved the problem. For some reason, though, I decided to clean-reinstall Firefox again. This time disabling hardware acceleration didn't help by itself, but when I also disabled WebGL it worked. Weird. Thank you for your help!

omotrl said

Thank you for your help!

My pleasure,   and :   well done   !

Just a short note about your extensions list.

I see in your extensions list that you have Go Parent Folder 2.9.1.1 and Show Parent Folder 2.1.1. Alice has withdrawn all her extensions, so I would advice to backup any extensions you have from Alice so you can reinstall them in case you need to refresh Firefox.

This could be a problem with the JavaScript JIT compiler since you already have disabled hardware acceleration and your extensions.

In Firefox Safe mode these changes are effective:

• all extensions are disabled (about:addons)
• default theme is used (no lightweight theme)
• default toolbar layout is used
• xulstore.json is ignored
• userChrome.css and userContent.css are ignored (chrome folder in profile folder)
• JavaScript JIT compilers are disabled
• hardware acceleration is disabled (Options/Preferences -> Advanced -> General)

• plugins are not affected
• preferences are not affected

So how is JavaScript JIT compiler disabled, by disabling WebGL?