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.