Hi, Arnold72:
Countless topics here (can't seem to find the topic links ATM

), at the
Mozilla support site, at the Adobe support site, as well as at the
MZ forum &
Adobe forum (and
elsewhere) over the past few weeks.
Short version: there were problems with Adobe Flash player 11.3 and Fx 13, esp for Vista and Win7.
(HINT -- Google search: "Flash Player 11.3 and Firefox 13")
The problems were (partially) resolved with recent version updates to both Flash (current version for Fx is 11.3.300.262) and Fx (current version is 13.0.1).
More complete fixes are due out with next Flash version and next Firefox version (v. 14, due to ship on July 17).
Best practice recommendation for now seems to be:
Close Firefox & all other browsers.
Uninstall Flash Player using the
Flash Player uninstaller.
Reboot.
Clean install latest version from the
official Adobe download site.
Note: Because the uninstaller will remove the IE/ActiveX version of Flash, too, you'll also need to download and reinstall that version, as well. The ActiveX version was not updated to the same version number as the plug-in version. Current latest release for ActiveX is 11.3.300.257.
This may or may not be your problem, but it's a good place to start.
There are other considerations, such as "hardware acceleration" in Firefox, that you might also investigate.
HTH,
daledoc1