[conradfr]

It made me go look in the menu and discovered the new Responsive design mode in FF15, it will be a valuable tool.

[fuzzix]

Looks quite useful, though I didn't see a way to customise the view port dimensions - it appears to lack common phone screen resolutions.

[dangoor]

There's a bug in bugzilla for that. Also, I think you'll find that the whole issue of "common phone screen resolutions" is actually painfully complicated. The iPhones, for example, report their resolution as half the actual resolution (in each dimension).

You can resize the view port to whatever size you want, even in Firefox 15. The resize handle is a little dark in 15, though.

[halvsjur]

According to the article, you can type:

    resize to 320 480

[lloeki]

Which is awesome, as I can finally drop my ridiculously contrived JS bookmarklets.

[auxbuss]

Also useful is the webmaster plugin which has a feature that displays a variety of page sizes in a new tab.