Firefox: Firefox has no shortage of keyboard shortcuts for popular commands. But if you really want to control everything about your browser experience, Customizable Shortcuts is a Firefox add-on that lets you set your own commands for just about any feature.
Once installed, you can manage shortcuts from the Firefox preferences/options window, where the add-on has inserted a new "Shortcuts" tab on the right. Click it and you can customise any of Firefox's built-in add-ons to fit your workflow.
Customizable Shortcuts even digs into your installed extensions and gives you the option to set shortcuts for extension features. I was surprised to find it offered me a keyboard shortcut to bring up XMarks' settings dialog. Customizable Shortcuts works anywhere Firefox does.
Customizable Shortcuts [Add-Ons for Firefox]