Hi! Does this problem appear on many different sites or just one?

Do you get this error reproducible on specific web pages?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

It always appears when I start the computer and, occasionally at other times.

I get that error message whenever I open a new window. I click OK and everything works fine, but nothing works till I click OK. I can live with it, but it's very annoying :(

Worse is the fact that when I open the Add-ons manager I cannot see any of my extensions or get any new ones, the Get Add-ons page continually saying Loading...

Appearance, Plugins and Dictionaries work fine.

this sounds like the files in your profile folder that store the information about your extensions might have become corrupted. please go to firefox > help > troubleshooting information > profile - show folder... then windows explorer should open up your profile folder. in there look for the files named extensions.ini, extensions.sqlite, extensions.sqlite-journal (and in some cases also extensions.txt, extensions.rdf). delete or rename those files and quit firefox, they will be regenerated the next time you launch firefox.

also see Corrupt extension files

So far so good, thank you very much. I was even able to find an update to an extension which is not compatible with my FireFox update :)

I still get the stringify error though :(

P.S. In my History tab, any idea why Recently Closed Tabs and Recently Closed Windows do not show anything?

Make sure that you do not run Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Cookies > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

Also check that session restore is enabled.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session

I never use private browsing, though I double-checked. Likewise session restore is set up, as one might expect. Still no joy :(

Quote: P.S. In my History tab, any idea why Recently Closed Tabs and Recently Closed Windows do not show anything?

Is this only happening if you close and restart Firefox or reboot the computer or also when you close a tab or window?

It's funny you say that, as I always log off the PC without exiting FireFox. In the old days closing FF first used to reset everything so I got into the habit of logging off with it still open. Today I had to close it to install a program and now my history works fine :)

Dare I say that this problem has disappeared?