Using Eset 15.2.11.0.
For last few days I wasn't able to update or install any new Firefox addon. Strangely, the issue existed on my desktop AND on my laptop.
Every time, I was trying to install new addon, I was getting message about addon file being corrupted.
I tried all, I could, refreshing FF, creating new profile, even clean Firefox installation and nothing was helping.
Thanks to a reply in FF forum, I realized, that the only common thing between my desktop and my laptop was Eset.
Lo and behold I went Settings>Advanced Settings and I clicked on "Default" button.
After restarting FF, all is good. I've been scratching my head since last night.
Broni received kudos from Securit in Secure all browsers option prevents installing extensions