On Ubuntu 9.10 and up, Firefox is slow. It's one of the biggest complaints I hear from new users of the OS and, I have to admit, it was the only thing that didn't work very well when I first installed my new 10.04 system. Perhaps the most confusing part of the problem is that it seems pretty random. Some web pages will load fine and quickly while others will just sit there and hang forever.
While many people on the mailing list and forums bang their head for hours (or even days), the solution is actually pretty easy and it mostly revolves around your hardware's support for IPv6. Some hardware supports it, others don't. If you're having the problem, yours probably doesn't - or doesn't support it well and you need to both disable and enable a few things.
Here are the steps. It's probably best to document your current settings in case something borks so you can revert to your old only half-broken settings.
1. In the Firefox location bar type 'about:config' and press enter. This will bring up the configuration options screen. There are a lot but, thankfully, you only have to change a few of those options.
2. In the 'Filter' box, search for 'network.http.pipelining' and change its value to TRUE by double clicking it.
3. Next, search for 'network.http.pipelining.maxrequests' and change the default value to either 8 or 10. I've noticed very little difference between 8 and 10 so the real value doesn't matter much. I set mine to 9.
4. Now search for 'network.http.proxy.pipelining' and change its value true.
5. Next, find 'network.dns.disableIPv6' and change it to true.
Now, restart the browser and you should notice a drastic change in page loading time.
There are a few people who believe you can get away with changing only the last entry. This didn't work for me at all and I didn't notice any change in performance at all. I had to change most of the entries to get it to work.
Also, there's some debate about enabling network.http pipelining. According to this article, you could experience problems with servers who don't support piplelining. I made this change earlier today and, so far, I haven't noticed any problems. But, just to be sure, you can probably safely skip that one if you're uncomfortable with it.
That's it. Firefox is now ready to power your web experience in proper fashion. Enjoy!