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Download the source zip and extract to a folder.
In Chrome got to chrome://extensions/, enable developer mode in the upper right corner, then use the "Load unpacked" button and select the source folder.
Go to (https://addons.mozilla.org/en-US/firefox/addon/view-commit-bitbucket-server/)
If that isn't accessible yet you can download the source zip.
Then go to about:debugging#/runtime/this-firefox, click "Load Temporary Add-On..."
and select the zip file as a temporary extension.