I hope this might help some others having this problem.
I also had this problem -- both Firefox and Thunderbird were nearly unusable because of the very long loading times, and long times to fetch web pages in Firefox. I narrowed the problem down to the eSet Internet Security firewall but didn't want to disable it altogether. So I went to the site shown earlier in this thread to download an earlier version and selected v. 14. I uninstalled Internet Security 15.0.16.0 and tried to install the version I'd downloaded, but got a message that the latest version was already installed. So I downloaded and ran the eSet uninstaller tool. Then I could run the installer I'd downloaded. And it installed v. 15.0.16.0, and not 14! But after all the fuss, Firefox and Thunderbird are working normally again, with 15.0.16.0. I believe I agreed to the automated use reporting the first time I installed 15.0.16.0 and not the second, but otherwise I did everything the same both times. LiveGrid was and is off.
I'm running 64 bit Windows 10 Pro, and the latest versions of Firefox and Thunderbird.
