I have run into the cursor bug before and it maybe due to the custom upgrades done by me.
A few things about MF that may be a part of your problem. There are two versions of PHPBB: the new PHPBB3 and the legacy PHPBB2 which many prefer because of the

bloated

features of PHPBB3 (and paid forums like vBulletin and Invision). MF still runs on a heavily customized and "super" version of PHPBB2 which I had to modify into running on the latest version PHP and MySQL while incorporating some features of PHPBB3. In other words PHPBB2 + PHPBB3 = mfPHPBB.
What does this have to do with your cursor issue?
I suspect it is a memory issue of some sort. When posting there is a
Topic Review window below the post box which features every post in the entire thread at one time - including images and video. This means whenever someone posts in a multi-page megathread like the Transformers for example, your firefox memory footprint inflates (mine is at 1gb as I type this) thanks to all of the multimedia content. Pulling all of that data up at one time while posting a mega-thread such as Transformers probably isn't too good for the database either, so it maybe a good idea to remove "Topic Review" or make it optional.
So is there any thread in particular that does this or is it all of them at this time?