So, I believe I have tracked it down. It also crashes in Chrome, but only the plugin so the browser stays active. In IE and Firefox it causes either the browsers to increase memory footprint until they either crash or hang, a dependent process to do the same(like plugin-container), or both . Culprit is good old Adobe Flash, which I believe is being used to run the ads in the top left of the forum window...
Have checked and am running the latest plugin. Also checked Adobe and there are a number of bug reports on browser crashes (I found around 1800 on a simple search) - see
here.
Until there is an updated plugin, think the answer is use chrome, or keep closing/opening your browser of choice.