Users have been reporting this problem much, much more frequently in the past several weeks. I wish I knew the cause, but (admitting that I can't read every thread), I haven't seen a good explanation yet.

In case this was caused by a locked file:

You can try the following:

(1) Exit/Quit Firefox and after 30 seconds or so start it back up again. This would hopefully work around any process that runs at Windows startup which might be checking the files (I'm looking at you, CCleaner and friends).

(2) For nerdier users: I found a post describing how to use Windows' Resource Monitor to try to figure out what program has opened a "handle" on the file (filter by places): https://superuser.com/a/643312

(3) Sometimes you may need to do a normal shutdown and/or restart of your system. Restarting Windows can help by freeing up locked files, completing any deferred updates, and resolving temporary file permission issues.

Check for a corrupted file:

If that didn't help, could you let us know what you get when you run the "Verify Integrity" diagnosis and repair function. That's here:

Open Firefox's Troubleshooting Information page using any of these methods:

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

Near the bottom, there is a section titled Places Database -- that's the file which stores history and bookmarks. Click the button labeled Verify Integrity. There may be a 10-15 second delay before results appear.

If all is well, the report that appears below the button should start with:

> Task: checkIntegrity
+ The places.sqlite database is sane
+ The favicons.sqlite database is sane
> Task: checkCoherence
+ The database is coherent
> Task: expire
+ Database cleaned up

et cetera

Does yours say the databases are sane and coherent, or something else? You can select and copy the report, and then paste it into a reply for review and comment.

Thanks for taking the time to respond. I'm so overwhelmed lately that it's taken me a while to get back to you. I'll keep your advice to refer to if needed, but so far I've found that shutting down and restarting seems to bring things back. This is a crazy situation! My difficulties have been exacerbated by installing a new browser that's causing endless interference with my online life.