https://github.com/mozilla-mobile/fenix/issues/417 it was requested back in 2019 for Firefox and is listed as a known issue on Mull https://divestos.org/pages/broken#mull it seems like Mozilla has no interest in implementing it. If you’re rooted you should be able to access the file though

I recently had to transfer phones and do this.

I don’t think there is a way to export a file as usual.

What I did was create a Firefox account using a fake/alias email through SimpleLogin just to backup the bookmarks. If you don’t have any privacy concerns then just using your usual email to create the account and logging in to Mull/Firefox that way should do the trick. Granted, I don’t really know how well that worked since I haven’t logged in to restore them, because I didn’t intend to, just wanted to save them. But I think that’s the only way on Mull. I hope someone else knows a way though.