HI Pete. Generally the JavaScript compatibility has improved a lot between browsers with, I believe, Safari being the worst. The problem is JavaScript keeps evolving, so slowly old browsers will break. Really pages should test for capabilities rather then having a list but people are lazy or just don't know better.
Then there are sites like Github that seem to stop working for no good reason, except I guess being a Microsoft property now. In the case of Github, I just changed to the next ESR version, FF52, SM 2.49 and it started working though I guess there might be subtle breaks.
Until we get a newer browser, it is worth trying different user agents to see if it works.
Edit
OTOH, just tried FF38.8 as FF52 and still no captcha here so actually needs a newer browser