If you want to upgrade to Firefox 4, and you use some extensions (add-ons) that you can't live without, make sure you install the latest version of Mr Tech Toolkit. Right-click this link and save the xpi file to your computer. Then open your add-ons manager in FF4 and click on the little cog wheel in the top right and choose to install from a file.
To make it so you can use non-compatible extensions: in the Add Ons manager, click on the Options for Mr Tech Toolkit, Miscellaneous tab, XPI Install Options, UN-check Enable Addons Compatibility Check, and make sure to check Allow web sites to install software (if you want to be able to install extensions from different web sites).
Note: sometimes using old non-compatible extensions can make things buggy, so, use at your own risk.
For more tips on customizing the Firefox 4 layout, see: