Usage

Use Ctrl+Space or click the Tab Finder button in your Toolbar to display a list of all of your open tabs across all of your browser windows.
Type into the text input to filter tabs so that only those whose page titles or URLs contain your input are listed.
Use the Up/Down arrow keys or your mouse to select a tab.
Hit Enter or click a tab title to go to that tab!

How to change the keyboard shortcut

From Firefox's main menu, choose Add-ons or hit Ctrl+Shift+A (Command+Shift+A on a Mac)
Click the gear button at the top to show the Tools for all add-ons menu
Click Manage Extension Shortcuts
Click the text box in the Tab Finder section, and type your shortcut
You're done!