I've found Blackberry browser to be so utterly broken, that testing in Firefox or even IE doesn't make sense. Netscape 4 seems like closest approximation.
If you want achieve decent experience on BB, then you have to get a BB simulator. It's available for free from RIM website (you need to click through a few forms).
If you just want to test under a mobile browser, than try Opera Mini simlator on-line or desktop Opera in Small Screen view (get version 7.6 or 8.0 from archives if you want exactly the same engine as used today on smartphones).
Or add html {max-width:240px}
to your styles and for testing serve them with media=screen
rather than handheld
.