Focus works out of the box with Safari, Chrome and Vivaldi. For other browsers like Firefox, Opera, Brave (and other browsers that support WebExtensions) we offer an official Focus extension.
Installing the extension in Firefox
To install the Firefox extension, visit the Focus extension releases and download the latest Firefox extension (should be named similar to focus-extension-firefix-x.x.x.xpi).
In the Firefox menu bar go to *Tools > *Add-ons. Click the settings cog dropdown menu and select Install Add-on From File...
Select the downloaded focus-extension-firefix-x.x.x.xpi extension and select open. Focus will now be installed.
Installing the extension in other browsers
To install the extension in other browsers requires a bit more work---the browser must support WebExtensions. Many browsers like Opera and Brave support this standard. Visit the Focus extension releases and download the latest extension zip file (should be named similar to focus-extension-x.x.x.zip).
In the extensions panel, click Load unpacked extensions (you may have to enable Developer mode first) and select the Focus extension folder. Focus will now be installed.