Did you replace the xulstore.json file or remove it? Replaced it with what?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

I just tried starting in safe mode, and I still have the same problem. To answer your other question, I deleted xulstore.json and restarted Firefox, which automatically replaced the deleted file with a new one.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

First of all, I really appreciate your help! Unfortunately, the suggestions didn't solve the problem. I went to Places Database and clicked on Verify Integrity. This is what I got:

> Task: checkIntegrity - Unable to check database integrity > Task: invalidateCaches - The task queue was cleared by an error in another task. > Task: checkCoherence - The task queue was cleared by an error in another task. > Task: expire - The task queue was cleared by an error in another task. > Task: vacuum - The task queue was cleared by an error in another task. > Task: stats - The task queue was cleared by an error in another task. > Task: _refreshUI - The task queue was cleared by an error in another task.

Then I restarted my Mac in safe mode (thanks for the link that told me how to do it). After booting Firefox I tried to bookmark a secure site. (I'm not sure of the definition of a secure site, so I just tried my bank, figuring that it would meet the criteria.) No luck with the bookmark. Then I tried to bookmark the site I originally wanted to bookmark, but it didn't work. Lastly, I went back and tried to verify the integrity of the Places Database again, with the same results as before.

It's never good when the integrity check can't run.

Could you review the following article. You could try the somewhat drastic approach described in there which is to remove/rename your Places database and have Firefox generate a new one using a recent bookmarks backup.

Fix "The bookmarks and history system will not be functional" error message

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

You can check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• use "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page

See also:

• http://kb.mozillazine.org/Locked_or_damaged_places.sqlite
• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

If "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page cannot repair places.sqlite then remove all places.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created
• http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can try to do some maintenance and cleanup with OnyX.

• OnyX: https://www.titanium-software.fr/en/onyx.html

Replacing the places database did the trick. Thanks so much for your help!

Replacing places,sqlite worked. Thanks for helping!