Integrate with Firefox's new profile manager

[dotproto]

Firefox is currently testing a new profile manager. As of Firefox 138, this experience "is rolling out to a Release test group in 138." Since this seems to be positioned as Firefox's new system for managing profiles in the browser, web-ext should be updated to support profiles that developers create with it.

[Rob--W]

Do you have any indication that it would not work? web-ext run creates a new temporary profile by default, but allows cloning an existing one for a given path via --firefox-profile (and even persist changes with --keep-profile-changes, but that is not recommended).

The end-user documentation for profile management is at:

• old: https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles
• new: https://support.mozilla.org/en-US/kb/profile-management

and Firefox source documentation is at https://firefox-source-docs.mozilla.org/toolkit/profile/index.html

From these, I don't see an obvious sign of the feature being significantly different.

Can you share the test case where web-ext would be unable to achieve the desired behavior?