OK, so the override value shouldn't be set. It's, erm, overriding the standard user agent identifying MicroB to the webserver with just the string "Maemo Browser". So the store can't tell whether you've updated your firmware or not. Dunno how it got that way, but just fix it by resetting the value.

Sadly you can't just right-click in about:config like you can on desktop firefox, nor will setting the value to empty work, so you can either (not recommended) edit ~/.mozilla/rtcom/prefs.js (you may need to kill browserd beforehand) or navigate to about:config, then in the URL bar, enter the following exactly (case sensitive, single line, and note different square/rounded brackets):