At first I thought this was some kind of config issue, but seems to be browser incompatibility. I have RStudio Server running and the Terminal tab never loads in Firefox (59.0.2 (64-bit), macOS High Sierra 10.13.4). No problem in Chrome (Version 66.0.3359.117 (Official Build) (64-bit)). No special proxies or anything else I can think of. Just an annoyance.
You might consider reporting this to the ide team as a potential bug. They'll need a bit more information, but others might have the same problems.
Thanks, wasn't aware if the right place! Will post to GH