Firefox has always been one of our favourite browsers for Blaq and with the release of version 4 this week it keeps getting better. We've been testing this update with Blaq and so far haven't come across any problems with Blaq's functionality. However, printing could be an issue for now.
When printing anything in Blaq, a PDF is generated and depending on your browser setup, the PDF is downloaded or is previewed in another browser tab or window. If you're using Firefox on a Mac the PDF plugin at https://addons.mozilla.org/en-US/firefox/addon/pdf-plugin-for-firefox-on-mac-/
works great, but it doesn't yet work with version 4. If you're using Firefox on a Mac for Blaq, we'd recommend waiting until this plugin is updated before upgrading.
We've tested the Windows version of Firefox 4 with Blaq and printing works fine, so there should be no problems with upgrading to this version.