ASUS TF300 Andriod 4.1 Firefox 17. .
This can be caused by one of files that contain details of installed extensions becoming corrupted.
Close Firefox, go to your profile folder and delete extensions.cache, extensions.ini and extensions.rdf
Next open the extensions sub-folder within your profile folder, if there is a folder called staged-xpis delete it.
Firefox > Exit and reopen it.
Is this about installing extensions on Firefox Mobile?
Did you spoof the user agent to get desktop pages?
If this is the case then you may have to disable this temporarily to be able to install extensions for mobile.