This setting is a classic in the "tweak Firefox" blog posts.
Increasing connection count will make HTTP 1.1 websites faster, but they'll also cause random rate limiting errors if you increase the value too much. How high you should set this value is up to you, but it's one of the first things you should reset when websites start acting weird in Firefox but not in other browsers.
This is good to know -- I primarily use FF and couldn't narrow down why my site was bugging out but running fine on other browsers. Glad its not a compatibility issue.
Increasing connection count will make HTTP 1.1 websites faster, but they'll also cause random rate limiting errors if you increase the value too much. How high you should set this value is up to you, but it's one of the first things you should reset when websites start acting weird in Firefox but not in other browsers.