Below is a screenshot of the BBC homepage running on Fennec 7 (UA string: Mozilla/5.0 (Maemo; Linux armv7l; rv:7.0.1) Gecko/20120205 Firefox/7.0.1 Fennec/7.0.1).If you poke about a bit with Fennec, it can be made to play flash content (apparently flashplugin can cause crashes though). Version 7 also supports WebM; though the playback is in fact quite patchy because it seems to force 360p, which is too much for stock kernel.

I tried these strings in the settings UI for Hide User Agent, which affects microB only:
1. Desktop equivalent to microB: Mozilla/5.0 (X11; U; Linux armv7l; en-US; rv:1.9.2.3) Gecko/20100423 Debian/5.04 (lenny) Firefox/3.6.3)
2. Fennec (as above): Mozilla/5.0 (Maemo; Linux armv7l; rv:7.0.1) Gecko/20120205 Firefox/7.0.1 Fennec/7.0.1)

Neither made a difference so it must be the layout engine using out of date terms. I'm going to try to "drop-in" Fennec's Gecko/XUL-runner into the microB directory. Hopefully the phone won't die, because of all the underlying OS stuff that depends on the browser, so I'll be able to report back some useful results...

Aside:
Fennec 7 is quite good. Unfortunately, its UI is not designed for devices (a) that have hardware keyboards, (b) whose screens are high dpi resistive or (c) that don't have a lot of RAM - i.e. the N900! Forms and media-playback aren't reliable but general browsing is good. Sometimes if you need to view things and neither microB nor Opera can handle it, Fennec is worth a try.