If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Troubleshoot Firefox issues using Safe Mode start window to disable all extensions.
You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")
Re: Why does my mouse pointer disappears when hovering over Firefox?
it seems that flash player is causing this problem because when i disable it, everything is fine. i even tried to install older versions of flash player (9.+) but pointer still starts to disappear. i'm using Windows XP. it happens on the both of my computers and it started at the same time (during the last week) i don't know what to do next..