Hmm, that's annoying. I don't see that behavior on Windows.

Could it be a new incompatibility with one of your extensions? For example, an issue was discovered with Word Count Professional in this thread: Why does my copy text get overriden when I try to create a hyperlink in WordPress?

With a new profile it doesn't break. I've got AdBlock Plus, Firebug, InstantFox, LiveHTTPHeaders, ModifyHeaders, Multirow Bookmarks Toolbar PLus, NoScript, Open in Browser, Tree Style Tab, Web Developper.

None of them should have anything to do with the address bar.

Seems that InstantFox is the only one that's focused on the address bar. However, I don't see any reviews indicating a problem.

https://addons.mozilla.org/firefox/addon/instantfox/

I send a bug report to Instant fox! Thank you!