clear the inventory cache manually:
Open Preferences -> Network & Files -> Directories
Click the Open button to the right of cache location. This should open a file picker window.
Locate the .inv.llsd.gz file that starts with YOUR avatar key. Your key can be found in your profile, under your name.
The file will look like this: XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX.inv.llsd.gz
(Depending on your OS settings, you may not see the .gz at the end of the filename, but the file type should show as GZ, gzip, WinZip, or WinRAR, depending on what utilities you have installed on your computer.)
ESSENTIAL: Log out of Firestorm.
Delete the .inv.llsd.gz file located above.