You're not the only one. I'm having issues too but I'm using the SaveTiddlers add-on (similar principle). This started on January 26th, but it was right at the end of my work day and I didn't really have any time to troubleshoot. I actually completely lost my TW as a result of this but was able to restore from a backup.
When I make a change to a tiddler, I'm prompted to save the TW (with a save dialog), but the save for the filename is not what I originally called it, It's "readTiddlySaverInstruction38335972.html". If I alter the filename and try to save it in C:\Users\MyUserName\Downloads\tiddlywikilocations (replacing my existing TW) then I get a message "TW not in tiddlywikilocations within the download directory. using default download directory.". However this should be the default directory as specified in the add-on instructions. In addition, the initial save attempt with the "bogus" filename completely deletes the TiddyWiki file, so if you simply cancel out of the second save dialog, your file will be lost forever!
If I simply try to save the file with the "bogus" filename given above, I get no error message but a new save dialog with the appropriate filename and an overwrite confirmation message. Upon clicking "yes", the file changes appear to save. I've never had to do this before, TW with the SaveTiddlers add-on has worked flawlessly up till now
without prompting me to save every time. I don't know if something has corrupted or if an update in Firefox versions has broken something (I didn't pay attention as to when exactly I updated Firefox, it could well have been on the 26th).
I don't want to stop using TW but this change is problematic, so I'll probably search for a different solution if this isn't something that is effecting all Firefox users (maybe switching to TiddlyDesktop like the OP).