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:

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.

There's a link at the bottom for me: https://www.mozilla.org/en-US/firefox/new/

Assuming you want to download firefox. It seems to be integrated into the browser, there's no add-on or so for this that you would need to download separately.

Weird, I had to go into about:config, search for "browser.tabs.groups.enabled" (which was set to false) and enable it there.

It is built into the latest version of Firefox. https://www.mozilla.org/en-US/firefox/new/