Bugs present in Firefox 4's basic behavior that weren't in or were fixed in Firefox 3?

Question

As a web developer, I encounter some quirks between browsers so I do expect having some inconsistencies along the way.
However, I do expect that by 'upgrading' from Firefox 3 to 4, I won't bump into issues that weren't around in the version 3 branch.
Note that for this test I disabled all addons, so unless the Web Developer Toolbar changes the browser beyond the scope of what can be disabled by disabling the add-on, I'm expecting these issues to be Firefox itself.
# 1) Having an <img> tag with an empty src atribute causes the browser to cause unneeded traffic. This seems to be a re-introduction of Bug # 444931, available at https://bugzilla.mozilla.org/show_bug.cgi?id=444931 . Ironically, it was fixed in 2009 as I can confirm on FF 3.6.17. However, FF 4.0.1 rediscovered it and tends to try and fetch pages that aren't there. True, images should have a src attribute, but then again HTML should be well-formed too. And we know how that turns out in real life.
# When viewing the source of a page with memory cache set to false, FF 3.6.17 just returns the current page's source code, as present in the browser. FF 4.0.1 under the same settings however issues another (remote) page request, actually potentially retrieving a completely different page.
Potentially? Yes, because aside from the fact that the server can randomize content on pages, the actual request being sent by Firefox doesn't even include the cookie data(!). So, we end up with a different source code than the one that was resident in the display mode.

Answer 1

Hey there. I had the same problem and I noticed the little white box went away when I was using the foxtab add-on. Then it came back after I stopped using it.
I disabled foxtab and haven't seen the little box since. I think it's a glitch with the add-on.
Before completely re-installing your browser as suggested above, I'd check to see if you have foxtab and try disabling it first Best, -T