As others said, it will be built into Firefox, and is already part of the mid-April release, but apparently not yet enabled. When I updated to 137.0.2, it opened the following page:
> Starting in Firefox version 137, you can use tab groups to manage open tabs in Firefox by grouping them together and labelling them. All users should expect to see the feature by May 6, 2025.
https://support.mozilla.org/en-US/kb/tab-groups
> Starting in Firefox version 137, you can use tab groups to manage open tabs in Firefox by grouping them together and labelling them. All users should expect to see the feature by May 6, 2025.