XUL cache reenables itself in pFF 1.5!

Everytime i try to deactivate xul cache it reenables automatically on restart. I cannot deactivate it, so everytime it creates the XUL.mfl of nearly 2 MB.
everytime i try to set the "nglayout.debug.disable_xul_cache" line to "true" (via about:config or editing prefs.js) pFF changes it back to false.
This used not to happen with 1.0.x
Any way to turn it off for good in 1.5?

My extensions in pff:

All-In-One Sidebar 0.6
CopyPageInfo 0.17.0
DownloadWith 0.0.7+
Duplicate Tab 0.7
Linkification 1.1.6
ListZilla 0.7
Minimize to Tray 0.0.1.2005091606+
Mouse Gestures 1.0.4
MultiScrapBook 0.17.3
Paste and Go 0.4.3
Redirect Remover 1.0
Sage 1.3.6
ScrapBook 0.18.3
SessionSaver .2 0.2.1.029
Tabbrowser Preferences 1.2.8.7
TargetKiller 0.3
Translate 0.6.0.8