I dunno if the Windows version is like this but the MXML editor has "Mark Occurences" on by default. This is the hacked FB4 project; to turn it off go to the Preferences dialog and just type in "occur" and you should see all the options for that. I disabled all of them because I dont like the feature, but one of them made the MXML stuff go away (I should note that there is no explicit MXML editor option on there for this. Hmpf.)
Hopefully I remember to look this up or search for the option next time rather than hunting though every menu option in the list! : )