I went to the StumbleUpon site and tried to reinstall the add-on, but got this error:
StumbleUpon could not be installed because Firefox cannot modify the needed file.
My setup is a bit unusual in that I have a QSoft RAMDisk drive set up on my 4 GB system to utilize the extra 768 MB or so of PAE memory, normally inaccessible to Windows 7. I configured my system to use a folder on this drive, R:, named TEMP as a scratch space, and set up Firefox to use R:\ for its cache.
I discovered, however, that I'd forgotten to create a little script or batch file to create this TEMP folder on the R: drive. This meant that for some programs that might use the system's default temp space, it might fail, the folder R:\TEMP being nonexistent.
To get around this I simply modified my environment variables for TEMP and TMP to reference the root of the RAM drive, R:\. Now the StumbleUpon toolbar add-on installs successfully.
In case you encounter this error, make sure that you have sufficient space available in whatever folder you tell Windows to use as scratch space, and of course that it exists in the first place and is accessible.