Or are you referring to the user name containing special chars?
User names are mail addresses (mail@domain.com) and passwords can contain special chars (only those requiring the combination of keys with Alt Gr)
I use other forms on the site with jquery without problems
This problem does not appear by pasting the id and pass directly into the input field
My Operating system: ubuntu 16.04
It might be that Firefox now does not allow password forms being submitted via http. If you haven't setup your site to use https that could be it. I did test with a local server (Mac) which is also http of course but Firefox might make a difference here. However I think it should actually only warn in that case and not block right away. Sorry otherwise I have no idea especially if your install hasn't been changed.