If I log on to my 2850 (3.6.3 - latest version at time of writing) using Firefox (19.0.2 - also latest version as of writing) the UI is incredibly slow, taking around 9-10 secs to load each page, but if I use Safari (6.0.3) it is extremely snappy - essentially instantaneous moving between pages. Has anyone else seen this issue?

I haven't tried with other browsers at this time because Firefox and Safari between them suit my needs, so I haven't bothered installing any others. I also haven't tried from a PC at this time.

I think you will find Draytek only recommend you use IE for management.

In this day and age that really is very pathetic. It's not as if a router can need to employ anything that sophisticated as far as the HTML is concerned, so making it compatible with multiple browsers should be pretty trivial compared to the task that some websites face which work well on all the main browsers.

Luckily Safari works great, but it is frustrating to have to use a specific browser when I use Firefox for all of the rest of my browsing; others must face the same issue.

I just tied with firefox V19.0.2 is is really fast here and my 2850n with Beta 3.6.3_r34169 from UK download page (note i tried from my PC wired)
Maybe is an add on/plug in causing this or a security issue or try to clear all history and cache of the firefox