Not sure what your use case is, but Firefox’s containers might solve your problem with the older profile system: https://addons.mozilla.org/en-US/firefox/addon/multi-account...
That person does not want something new, they just want to use their existing workflow, which is possible, but inconvenient due to bad UX and ugly UI. These are their words, not mine.
As a user of both containers and profiles in Firefox I have to agree with them. The UX needs to improve and the UI needs to be prettyfied.
the add-ons are basically just the interfaces for you to access that core functionality.
If you don't want to trust the add-on developer just use the official one made by Mozila. It is the one i use.
https://addons.mozilla.org/pt-BR/firefox/addon/multi-account...
Additionally Firefix also have profiles as a core funtionality, it already had it before chrome even existed so it had it much longer then chrome had.. But again is mostly hidden away..
You can access it with the command below:
firefox -ProfileManager
But as with containers there are add-ons to make as easy as chrome to switch between profiles.. Although i don't think there is an official one by Mozilla..