Cursor falls behind text

This is an odd error that I have been getting using Firefox Portable and MobileFirefox before that. It's the oddest thing, but when I type in a field within Firefox Portable, it seems like occasionally the cursor "falls behind" for a character. So I'll be typing along and then it will fall backwards one space so that it in effect pushes one letter out of my text. Like if I were to try and type "I am a monkey" then maybe something like this would happen, "I am a mokeyn," because the cursor fell behind when I typed the 'n' and ended up shoving it to the end of the word while I continued to type........... This has actually happened about 30 times in typing this message, so it renders Firefox virtually unusable.

I actually switched away from MobileFirefox because of this problem, and I wasn't getting it in Firefox Portable until today. I was thinking that it might be the computer itself, but regular Firefox seems fine, as does IE.

I'm running Firefox Portable off my computer as well, so I know it's not an issue with my USB key not being able to handle the load.......... I'm so annoyed, I have no idea what could be doing this, and it's hard even trying to describe this freaky problem.