Firefox randomly putting localhost into text fields

I originally posted on Mozilla's help site, they referred me to here:

I'm using Firefox Portable. For some reason, in text fields, firefox will randomly "paste" (for lack of a better term) the localhost into a text field and act like it's pressed enter. It first started when I was typing, but now it can occur when I'm changing tabs and occasionally when I'm doing nothing at all. No other programs on these systems are doing this.

I'm using this for data entry, using a help desk web site and a sharepoint database, so when it does this, it does leave a permanent record in there, even after I've gone back.

Is there any way to stop this? I've updated the software and tried reinstalling. I don't have any extensions or anything added to the base install. It happens randomly, but more and more frequently. These are new Windows 10 computers (mostly Panasonic FZ-55s, some Dell Toughbook models), and I can't find anything on them that would be causing this issue, much less coming up with the localhost (127.0.0.1:) out of thin air.

Any help would be greatly appreciated.