What security software do you have?

Firefox would only create or keep a .BAK or other temp file when it can't rename the temp file because other software keeps an handle on it.

Windows Defender only. I have read that firefox can create a cookies.sqlite.bak file when some cookies are corrupted.

Possibly the Browser Console (Ctrl+Shift+J), which collects background error information, might have a message about it if you have it open at the time the problem occurs.

Here's my browser's console log just after going to twitch.tv. It creates cookies.sqlite.bak and cookies.sqlite.bak-rebuild files in my profile folder. Also now when I log into my twitch account, it logs out when I close the browser. Previously I didn't have to log in because the account was logged in.

Hmm, unfortunately there's no message about the database operations. I wonder whether that is handled in some other code.

But it occurs to me that we need to rule out an error reading your Twitch cookies, which Firefox needs to do to request a page from Twitch. After you restore your old cookies.sqlite, before going to Twitch, could you delete your Twitch cookies and site data using the Manage Data button?

Settings page > Privacy & Security > Cookies and Site Data section > Manage Data

Does that solve the problem, or generate the same behavior, or neither?

Oh wait, I see you already tried that. Hmm, I'm stumped what could be in the problem cookies.

Yes, I tried that and it didn't help. I think it might be a problem in this cookie database, but I don't know if I can somehow open it and manually search for errors. On the new profile this problem doesn't occur, however I would like to keep my existing cookies and the pages I'm logged into :(

Did you try to start with a fresh cookies.sqlite database (i.e. rename/remove the current version and let Firefox create a new file) ?

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be careful. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Yes, I tried this and it didn't change anything.

On the new cookies.sqlite this problem doesn't occur, but I also don't have access to the cookies and I'm logged out of all sites.

I don't know if there is an option to move the cookies to the new cookies.sqlite.

Ok, I managed to export the data from the old cookies.sqlite file to the new cookies.sqlite file and now it works fine. Thanks for your help :)

Thanks for reporting back. Did you find a convenient tool for that or did you have to use a SQLite browser?

I first used the SQLite browser, but have no experience with it, so I looked for Mozilla addons to export cookies to a txt file.

I used this to export old cookies to txt: https://addons.mozilla.org/en-US/firefox/addon/export-cookies-txt

Then I deleted the cookies.sqlite file from my profile folder and launched Firefox to create a new, clean cookies.sqlite file.

And then I used: https://addons.mozilla.org/en-US/firefox/addon/cookie-quick-manager/ to import the cookies into the new cookies.sqlite file.

I don't know if this method is safe, but it works.