Most likely it's some dumb rule that blocks local connections they've added recently, without caring if it's going to break something. Are there no settings you can tweak to allow these connections?
Hi
I found the same problem with my Firefox 98 installed without snap process. I do not know if we found the same issue, but I dealt with this difficulty, removing and installing again the Joplin extension in my updated Firefox version. Trying to check it if this resolve the issue.
OK, I had to learn about extensions to be able to debug it.
It was pretty straightforward - I had to open special page about:debugging, find joplin extension and then click on the J button.
It shown me that connection to 127.0.0.1 is overtaken by a proxy So I manipulated proxy entries and allowed that port to work locally. Voila! I have the extension working back.
It's an amazing day today, I really missed Joplin for last few weeks!