I was extremely frustrated with this issue until I found this:
From the Firestorm Wiki:
If you are crashing when you try to upload a texture, please try disabling VBO, as described above (below). Also, if you are on Windows and running Microsoft Skydrive, you will need to perform the workaround listed in the Comments section on FIRE-6186.
Disable VBO: Run Firestorm and before logging in, press Ctrl-P to bring up the Preferences window. Then click on Graphics → Hardware Settings tab. Make sure that “Enable OpenGL Vertex Buffer Objects” is not checked.
http://wiki.phoenixviewer.com/fs_run_crash
It works just fine with the SL Viewer, too. Now I can upload!