As SessionBox is currently not in the Firefox addon library, we host the installation file for it. Therefore when installing, you will see a notification stating this. That's totally OK and should cause no problem at all, however, sometimes Firefox is unable to properly install this type of extensions. Here's what you need to do now:

Step 1: Download this file from a different browser (not Firefox): 

Step 2: You can now manually install SessionBox addon on Firefox by simply dragging and dropping the xpi file into the Firefox addon page.

Getting into the Firefox addon library is a plan for us, however the very long time needed for that is not really something that is suitable for a continuously updated and frequently released software and can lead to extension and backend API mismatch. Please contact us, if you see any other errors when installing