Do you run ESET security software? Some version(s) of ESET have been blocking add-on installs, but I haven't been watching the status of a fix for that.

Here's a link to view recent threads: eset add-ons

I had turn off Eset, didint help. Next I unistalled Eset, doesnt work too.

Okay, here are two older issues:

(1) Connection issue

If you right-click the Add to Firefox button and use Save Link As, can Firefox successfully download the file?

(2) Verification issue

One reason for verification to fail is if your computer clock is set to a date well in the future or the past.

Thank you for answers. I had to much work to try solutions @jscher2000. I have automatic clock and date set. Few years before i had the same problem, didint see my clock was incorect. But with that problem, everything was good. Today i on my computer, to see that maybe work your first solution.

I dont know still what happend, One week i can't install my addons, now i turn on firefox and addons i was trying install, now are installed. I dont know how, i didint do anything. I only know that yesterday nothing work, today work everything.

Thank you @jscher2000 for help.

Ps. Sorry for my bad english. I live in Germany for few years nad my english little rusted. But still i understand better english than german, only problem with writing and talking. But reading still better.