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.
Were you able to find any more information on this?
Coincidentally trying to do this as well. I’m not sure if it’s possible to play HL mods without Steam anymore.