In case anyone's interested, I just wrote a little Greasemonkey script which helps to make up for the GOG forums' lack of e-mail updates.

GOG.com - Updated Thread Count in Title

Here's how it works:

1. You install it (obviously)
2. You open up https://secure.gog.com/forum and choose "Pin Tab" from the context menu
3. It'll refresh the page once per hour like an RSS feed would
4. Every time the page loads, it'll count the number of highlighted entries in "My favourite topics" and use Tinycon to put the count in the icon (as well as in the page title so Firefox's App Tab "something has changed" glow can work).

Now, you can just glance at the pinned tab every so often to see if any new posts are waiting.

Note: If you are logged out, it will show a red X and, if there are no posts waiting, it will leave the icon blank rather than showing a zero, so an icon without a count doesn't automatically mean it's broken. Check the tooltip for a "(0)" before reporting a bug.