Font sizes set in Preferences won't stick

Anybody know why my font sizes keep reverting to factory defaults in Preferences > Formatting? They’ll stick for a given session, and across a few relaunches of AppleScript Editor 2.3 (118), but pretty soon I’ll launch it, and they’re back to the defaults. It’s been going on ever since I upgraded from Leopard to Snow Leopard a few weeks ago.

Model: MacBook Pro (5,2)
AppleScript: 2.1.2
Browser: Firefox 28.0
Operating System: Mac OS X (10.6)

When you are talking about AppleScript Editor’s prefs: yes, that’s a bug (reported to be cured in 10.7).
Set fonts as desired, then lock the plist that stores them:

Keep a copy of the file in a safe place too. Some later updates and upgrades add more AppleScript formatting categories and replace the plist file even if it was locked. When this happens, quit AS Editor, replace the new plist file with the back-up, reopen AS Editor, unlock the back-up (if locked), edit the new categories, click “Apply”, close AS Editor again, and immediately lock and back-up the plist.

Since upgrading to Mavericks, my AS formatting has occasionally been reverting to the defaults without the plist file being overwritten. The only cure when this happens is to restart the machine. I haven’t been able to pin down the cause, but it seems to be associated with my sporadic attempts to learn ASObjC!

Many thanks, alastor933 and Nigel Garvey!