Rated 5 out of 5 stars
I've already tried and it didn't help. I don't use nightly tester tools, instead I use ignoring compatibility check (extensions.checkCompatibility.4.2a = false).This review is for a previous version of the add-on (126.96.36.199.1-signed.1-signed). This user has other reviews of this add-on.
I still can't reproduce it. I used the preference you said (extensions.checkCompatibility.4.2a = false) in Minefield 4.2a1pre (2011-04-11) x86 on Win7 Pro x64 with a clean profile with no AddOns (except New Old Menu) or themes (except default) and installed the version before yesterday's version bump of New Old Menu - and it worked. Weird.
Do you get errors in error console when enabling / disabling New Old Menu?