Weird Mouse vs. Firefox issue - rogue characters

Got a rather minor nuisance happening with my browser/mouse combo, and I’m hoping that someone here has seen something similar and can offer a solution.

I’m using Firefox as a browser and a Logitech MX 600 cordless mouse. Often when I use the “forward” and “back” buttons on the side of the mouse Firefox will stick a strange character into any input boxes on the website I am returning to. This is especially annoying when I’ll use a search engine and click back to revise my terms I’ll have to go out of my way to delete that rogue symbol.

This might not be limited to Firefox, but that’s where I always end up noticing it. My google-fu is weak so I’m hoping someone here can lead me to a solution.

Is it the same character each time? What happens if you use the keyboard shortcut equivalent of forward and back? Which I think is alt-cursor keys.