When I try to install the uBlock add-on for Firefox from this site : https://github.com/gorhill/uBlock/releases/tag/0.8.5.4-beta.0
Clicking the “uBlock.firefox.xpi” install link will result in a “this add-on could not be downloaded due to a connection failure to github.com” error after the download ends and it tries to install the add-on.
It only works if I temporary disable Avast.
This is a serious bug that should be looked into. 
Anyone able to reproduce please ?
Anyone please want to look into this ?
Are devs reading this ? Do they try to reproduce it, let alone trying to find to problem that is clearly caused by Avast Web Shield ?
Please ? 
Disable https scanning and see if it makes a difference.
And the download is not from Github, but from Amazon
Disabling https indeed made it work.
I never knew it is actually downloaded from amazonanews, since I never got that far.
So, seems like a bug in Avast to me.
A new avast version will be released later this month or next month (or at least that is the word on the street).
There should be a improvement in that version.
Main thing is that it is working now for you.
Just wanted to pipe in that this is still an issue as of mid April. I had to turn off the Avast shields to be able to complete a download from github (it was also a Firefox plugin, and I was downloading it using Firefox, if that matters).
The ssl scanning problem has been fixed in the latest version(s).
https://forum.avast.com/index.php?topic=169573.0