Not sure if it can help you, but try this method:

1. Open browser (MicroB)
2. Type into address field "about:config" (without quotes)
3. You will see information that you should know what you are doing, just accept it
4. Put into search box (on the top of that "page", it should be labeled something like "Filter") "general.useragent.vendor" (without quotes), no need to hit Enter
5. You should get one result, the one that is interesting for you: general.useragent.vendor (should be written in bold, at least on my phone it is)
6. Mine says "Firefox/3.5 Maemo Browser 1.7.4.8 RX-51 N900" so if you are on PR 1.3 you should have the same value. If not:
7. Note (write down maybe on piece of paper) your current value (if something goes wrong you can restore your current value manually)
8. Swipe on top of the screen to see cursor (you know... the white arrow, I guess you know how to do that)
9. Hit the line "general.useragent.vendor" in bold with the cursor, a window should open where you can change the text
10. Accept new text, close browser, re-open and check if it worked, if not (and text in general.useragent.vendor shows value which you just entered) maybe restart the phone. If that doesn't help - I don't know what else can you do.

Good luck