To record a performance profile in Firefox, open the developer tools and switch to the performance tab.

Click the "Start Recording Performance" button.

When you're done recording, click the "Stop Recording Performance" button.

When it's finished processing, click on "save".

This should save with a filename like profile.json.

You should be able to open that file in https://www.speedscope.app/.

To view it offline, you'll need to install speedscope via npm:

npm install -g speedscope

Then you should be able to open it like so:

speedscope /path/to/profile.json