Did you know you can tidy Firefox up, with VACUUM? No, I do not mean that thing you use to clean your carpets.  Here is a way to VACUUM your places database from within the browser.
Here are the steps you need to take:
1. Open the Firefox tools menu and click on error console.
2. Paste the following command into the code box as a single line:
Components.classes["@mozilla.org/browser/nav-history-service;1"].getService(Components.interfaces.nsPIPlacesDatabase).DBConnection.executeSimpleSQL(“VACUUM”);
3. Hit the “Enter” key or click the evaluate button.
The user interface will freeze for a [...]

Some have discovered a new project from the Mozilla Wiki documenting the first steps and timeline for Firefox to become a multi-process supporting browser.  The project in question, simply known as Content Processes looks to separate processes to display the browser UI and web content.
Here are the current initial goals:

Provide better application UI responsiveness
Improve stability from slow rendering or crashes caused by content
Improve performance, especially on multi-core machines

Now why do this, and what does it even mean?  [...]

We are getting closer to a final release for Firefox 3 and that is the sort of thing that just makes me all happy inside.  Not much is really known about the Firefox 3 RC2 release as far as what has been improved.  In a general nutshell it is said they fixed some reliability and performance issues.  For me?  It is just as awesome as Firefox 3 RC1 was, so thank goodness for little things.
You can download the Firefox 3 [...]

This is a ‘bit of news that might have been missed by some. It looks like Firefox 3 continues to gain praise for having a significant reduction in the amount of memory it uses over Firefox 2.
Over at Ars Technica this is what they are saying:
During intensive browsing with approximately 50 tabs, I have found that Firefox 3 generally consumes less than half of the memory used by Firefox 2.0.0.12. Firefox 3 is also snappier and more responsive when [...]

Firefox 3 has a lot of nice new features, the design is slick and it promises to be yet another Mozilla home run! Now a lot of people could really care less about those three facts because much of the Web is much more considered with how fast Firefox 3 is going to be. Well thanks to Ryan over at CybernetNews.com we have some numbers for you.
Using the SunSpider JavaScript Benchmark test for each of the browsers, here [...]

Which one gives you a better performance? Is it better to have multiple tabs open in one window or just have multiple Windows open? The Firefox Guru did all of the scientific work on this one, and has found out the results for us.
Over on Go Firefox!, an interesting question was posted: Which uses less system resources (memory)?
My theory is using tabs would use less resources then having the same sites opened in separate windows. My reasoning is [...]

Am I the only person out there that has not ever really noticed a big speed increase when using Fasterfox? The newest version of the extension is out, and it now works with Firefox 2. If you can, leave me a comment if you have really noticed a difference using this extension – because I am confused why I have not.
Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, [...]

The folks over at Download.com have a good guide when it comes to optimizing performance in the Firefox browser. Some of the recommendations are common sense, such as clearing out your downloads. Others might be something you haven’t thought of before, such as deleting extensions you don’t use too often. If your browser seems to be lagging a little here lately, then this guide is worth a look.
[Get Firefox Up to Speed Again!]