One some websites (most recently https://tinkercad.com) FF does not give me a selection of my saved username/password combination on the sign in screen. It works fine on most of my other sites, like a roundcube installation or any of the dozens of other sites requesting login/password, even the ones with autocomplete=off which I removed using a bookmarklet.
As I'm using both throwaway mailaddresses and randomly generated passwords, the save password function is essential for me. How can I make it work with tinkercad.com?
Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
Once you get the pop-up, just select "'Start in Safe Mode"
If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.
When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.
The problem is still there in safe mode. I've also tried clearing the offline cache and cookies as well as using an entirely new profile.
The built-in Password Manager may not work in cases like this.
You can use this extension to save the password, but most likely Firefox won't fill it either in that case.
But at least you can paste the password from the Password Manager.
@Waka_Flocka_Flame: As mentioned before, I tried it even with an entirely new profile, which did not have any addons.
@cor-el: Thanks for the hint, but as I mentioned, the password DID get saved, it just isn't inserted into the form.