Graaah! Just putting this out there in case there's someone listening or searching who may be able help with, or at least confirm, this. But it seems with a recent Firefox upgrade, Movable Type (5.12) has stopped recognizing when i've checked Category checkboxes. They're checked, you submit the entry, and then they're not checked and your post has no categories.

Old standbye Internet Explorer works fine. But i prefer writing my posts in Firefox. Despite the commercial that plays about 30 times every time we watch the Daily Show online, i still find IE to be slow and clunky.

I see MT has version 5.2 out as of last month. So i could try upgrading to that, although MT upgrades usually result in at least a day's worth of tears, and there's nothing of interest to me in the upgrade and no guarantee it would fix this problem.