Hello everyone and sorry if, perhaps, this is not the appropriate place to discuss this problem of mine (is it?).
I have Firefox 103.0 (64 bit) installed on Linux Mint 20.3.
I have been installing and using the Multi-Account Containers add-on for a few days, modifying it to my liking until yesterday, I liked it very much.
Yesterday, however, I noticed that one of the containers, moreover the one for Banks (which I was using for 2 banks and for Paypal–which opened automatically in that container and which I had set up to work with MozillaVPN), were having problems connecting (the place in the VPN I had chosen had no connection) and, since from the containers it is not possible to change the choice of VPN site, I had decided to delete the “Banks” container and recreate it from scratch without automatic VPN connection. However, I did not delete the contained sites from within it first (perhaps this was the fatal mistake that compromised everything!).
After deleting the container in question, I created a new container, however, calling it by another name “Banks and Money” (so “Banks” no longer existed) and right-clicked on the Bank Bookmark saying to always open in the “Banks and Money” container. When it opened, however, it would try to open again in “Bank” (nonexistent), then close again and immediately open again in “Banks and Money” to close again, and so on and so forth ad infinitum. At the time, the only way I thought it would help was to disable the Addon and try reopening without the Addon the site in question. And it worked. At the time.
When I re-enabled the Addon, however, all the containers I had created earlier by losing several hours in them were gone! I started creating them again, but when I got to the bank sites, they would not open and would do the same trick as before (they would open and close right away).
After an Account synchronization, all the containers I had before suddenly reappeared but, now, the bank sites would double open for me, and each one would cause two others to automatically reopen. Before I could do anything Firefox crashed!!!
I was so pissed off that, from https://addons.mozilla.org/it/firefox/addon/multi-account-containers/ (I was looking for a way to fix this) I angrily clicked uninstall, but that got me nowhere, the bank and Paypal sites kept not wanting to open for me (the tab would open but this would close again soon after). I tried reinstalling the Addon (maybe it was because I had uninstalled it from the site and not from the add-on manager?) but the problem remained. I uninstalled it again but the sites in question just won’t open. Not only that, even though I no longer have the Addon installed, when I click on “new tab” the list of Conteiners I had before appears.
Finally coming to the end of the story (excuse my verbosity) I ask you: how can I solve it?
Thank you all and regards.
PS: sorry for any mistakes, I am Italian and do not speak English 
Translated with www.DeepL.com/Translator (free version)