You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

I have, as suggested, found the localstore.rdf file, re-named it, exited Firefox and then restarted but that has not solved the problem

You need to delete the localstore.rdf file while Firefox is closed and not while Firefox is running.

When the sidebar is open when you close Firefox then this sidebar should reopen automatically the next time.

Thank you cor-el. I have now done all that in the correct order(!), but Firefox is still not remembering my setting for the side bar. It always opens without the sidebar and I have to use View>Sidebar to get it to appear.

Try to open a new window and close the current window to see if the sidebar stays open in the new window.

No I am afraid it doesn't.

Incidentally I should perhaps say that my version of Firefox is now 33.0.3.

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

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

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Thank you Cor-el.

Starting Firefox in safe mode sometimes results in it opening with the sidebar in place - but sometimes it doesn't! And I don't understand why.

Disabling all the add-ons and extensions manually doesn't seem to make any difference - the sidebar still does not appear on opening - though of course I can get it to appear by clicking View>Sidebars.

You advise above NOT to use the Reset button in the Safe mode window. What would happen if I did that? Would I lose all my present settings, bookmarks etc? Or is it worth a try?

An other option is to create a new profile to check if your current profile is causing the problem instead of of using a reset that can't be undone easily. That way you can go back to your current profile if the new profile doesn't help.

See "Creating a profile":

• https://support.mozilla.org/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
• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile

Geronimo!!! Creating a new profile has worked and all seems to be back to normal.

Thank you very much indeed cor-el. I am very grateful to you for your help and your patience.

Postscript

After sending the above I started to reinstall some of the Extensions which I had used before. I found that after I had installed the Easyfundraising donation reminder, the bookmarks sidebar no longer appeared. I have uninstalled that, and once again replaced the profile files from my Transfer folder and all is now working again as it should.

So it looks as though the Easyfundraising reminder is the culprit. And in fairness Firefox did try to stop me installing it - I should have taken more notice.

Nonetheless I have been using that reminder toolbar for a year or more and it is only in the last couple of weeks that it has caused any problems. Presumably some update to Firefox has caused the two to be no longer compatible. Maybe I should no longer allow automatic update installations!!

David