Dpkg file firefox download

I've recently installed a fresh xubuntu and I'm having a strange bug on (it was already there in the preceding version )

Every time I download a file, even if it's a tiny tiny file, when the download popup appears to choose the destination and accept the download, the whole firefox process freezes for about 5 seconds waiting for something in the background, and then only I can accept (or not) the download and keep browsing.

EDIT: Actually it doesn't fully freeze, it's just ultra laggy, and display gets weird if I try to move the pop-up.

What I've tried:

• I've tried uninstalling with but for some reasons all my tabs, bookmarks, config and extensions were still there after reinstalling
• I've tried to then check all files used by firefox with but some must be missing because I didn't see any config files left but still, my config was still there after reinstalling
• So I thought okay whatever, this may be caused by an extension or some options so I tried disabling all extensions -> no change
• I've tried disabling hardware acceleration in options (after all I'm not sure what it does so why not try it out) -> no change
• I ran and tried downgrading to the only other version in the ubuntu bionic repositories : -> it just crashes when starting the program

At this point my lack of sys skills (even configs resist me and won't get deleted wth) tells me to ask for help because I don't really know what's the problem and where to look for. I could try to manually install older versions of firefox but if I could fix this problem while staying with an up-to-date browser that would definitely be a plus.

Thanks for any help you could provide.

EDIT2: Running firefox from the shell gives no meaningfull errors when the bug occurs. I've seen there is an option to run firefox in debug mode in gdb but I'm not sure I want to try and look for the error there, maybe later.