Something strange started happening a few weeks back.
When I clicked on a link in an Email, first time after Firefox was opened it opened the page correctly in Firefox. Thereafter I got a pop up that Firefox was not responding. Firefox itself continued to work normally and would accept the link when I copied and pasted it into the browser. This was happening on both my Windows 7 PC and my Windows 10 PC, and was not restricted to a particular Email client. Another symptom was that when I clicked on a link with Firefox not running, causing it to start up, it would flash up (something?) which immediately went away then bring up the Firefox window, but Firefox would be "behind" the mail client window, where normally a new window would be on top. I had the latest version of Adblock (3.5) and auto update on.
After a lot of research and messing around, I tried starting Firefox in Safe mode, where the problem did not occur. That seemed to suggest an add-on as causing the problem, and I found that disabling (NOT removing) Adblock also made it go away. What's interesting is that after I enabled Adblock again the problem (including the "double start") has not recurred. Also Firefox seems to be a lot faster in its responses.
Edited to add ...
I rebooted and it came back. Here's a better description.
1. Problem exists.
2. Disable Adblock. Problem exists.
3. Restart Firefox. Problem goes away.
4. Enable Adblock. Still no problem.
5. Restart Firefox. Problem returns.