Thanks for your answer!
I've got one error message during review regarding greasemonkey:

Error: unsafeWindow.Function.prototype is undefined
Source: file:///.../Mozilla/Firefox/Profiles/.../gm_scripts/kanjikoohiisubstitutekey.user.js
Line: 425

Also on line 802 in the study area.
Hope that'll help...

Update:

The same for gently expire:

Error: unsafeWindow.Function.prototype is undefined
Source: gm_scripts/kanjikoohiigentlyexpire.user.js
Line: 45

Thanks a lot for your help, Woelpad. Anyway, I downloaded the last version from the Substitute keywords thread and now everything is working ok again. I am even using both substitute keywords and change font script without any problem.

　By the way, xul migemo is working again! (yeah! )

Now the only important add-on that isn't working yet is theTorrent Finder Toolbar. (boo! )

Well, an update to the newest script solved the problem...
Thanks for the "advice", Transtic :-)

And sorry for the trouble, woelpad. I really missed your scripts the two days I couldn't use it now... they are a great help!