1. Firefox with the Classic Theme Restorer add-on. This would be the simplest solution (duh) but not my preference. I don't like installing extra add-ons if not required and the classic theme restorer does not fix all issues related to Australis/new Firefox releases.
2. Pale Moon. Used it for several weeks on an old Ubuntu install and plan to keep it as that system's primary browser. Runs stable and snappy enough. Based on Firefox with supposedly *optimized* code for newer hardware (requires an SSE2 capable processor). Less of an installation footprint than firefox but does not support old processors. Runs all of my favourite Firefox add-ons, stable, easy to set up, traditional interface, lots of configuration options, etc. Really like this browser. More info can be found here: http://www.palemoon.org/info.shtml. Doesn't follow Firefox's quick release cycle - thank goodness. Syncs with Firefox/Iceweasel no problem. Does not require installation and can simply be run from the executable or via symbolic link. When first run creates a new profile in ~/.moonchild productions so it doesn't interfere with any Iceweasel/Firefox install and can be run concurrently, which came in handy when setting up sync and select about:config preferences. Firefox based so it can still utilize any preferred userChrome.css script.
3. SeaMonkey. Not sure why i didn't think of this earlier and nobody mentioned it in the thread - not very popular i guess. I first tried SeaMonkey quite a while ago but don't think it was compatible with Firefox sync at the time. It's based on mozilla code. Still like it and am presently running it instead of Firefox on my oldest computer, which does not have an SSE2 compatible processor and can't run Pale Moon. SeaMonkey seems to run slower than Firefox, especially when processing scripts, and this observation appears to be supported by online performance comparisons. It is, however, very stable and just feels solid. Not compatible with all Firefox add-ons but all of my favourite add-ons still worked with one exception (autoclose bookmark&history folders). It is compatible with Firefox sync except the latest release (version 2.26) broke sync indefinitely with no fixes planned for releases 2.26-2.29. This was a deal breaker for me but was able to work around the issue by installing version 2.25 first, setting up sync and then upgrading to version 2.26. Now sync works just fine. Similar to Pale Moon and Iceweasel, SeaMonkey is also highly configurable and has a very simple logical layout. It creates a separate seamonkey folder in ~/.mozilla and can also run concurrently with Firefox. SeaMonkey does not require installation and can also be simply run from the executable or via symbolic link and still utilizes a userChrome.css script if desired. Just use it as a browser but it's actually a suite that includes a built-in mail and newsgroup client as well as an html editor. Definitely worth trialing. Even though it's a little sluggish i plan on continuing with it and it may one day be my primary browser. More info here: http://www.seamonkey-project.org/
Sorry for the long post but i thought it migh be an insightful read for anyone interested in alternative browsers, especially for those who don't have as much free time on their hands to test browsers
i think it is almost impossible to find uglier skins 
