I'm using LinuxMint 71.3 with Firefox 42.0. The black bar is OK, but with an open thread, the thread title line in green at the top of the thread is slightly too high, and overwrites the line saying 'All Forum Topics….'
Its always been like that with Linux, both Mint and Ubuntu, and since any changes to the website will never happen, I just live it, like Jason Udall.
I am also running Adblock2, Privacy Badger and a video download addon, but even without these, the problem is still there, so I also think it is a site problem. I don't think it is anything to do with Java, or a corruption thereof, partly because I have always seen the problem since I started looking at the ME site, and also because in recent days I have updated four computers from older versions of LinuxMInt or Ubuntu to the latest Beta release of LinuxMint (17.3) – a laptop, and three desktop machines, all with different specifications for CPU, memory etc. The problem was present on all the machines before update, and is still present after update. The only guess I can offer is that Firefox on Linux handles its screen displays in a different manner to Windoze/IE etc, and that possibly some parameter governing text size or location is either undefined or incorrectly defined.
(Before someone flames me by saying Linuxmint 17.3 is a Beta, still under development, so it can easily be blamed on that, I will point out that exactly the same problem existed in all releases of LinuxMint 15, 16 and 17, the long term support version).