As people have said containers is amazing. But in the scenario where you really need 2 completely separate browsers and don't want to use different operating system accounts (I've done this in the past).This extension has worked well for me https://addons.mozilla.org/en-US/firefox/addon/profile-switcher/ It's not ideal that it's an extension but it does do its job
Firefox
A place to discuss the news and latest developments on the open-source browser Firefox
Did you try containers? Though truth be told I am not amazed by any of these solutions too.
Containers are not the solution for everything.
Here is an example, I use the website aliexpress. I want to browse with multiple identifies, from various countries, different account and I also need to access with no cookies and no login, completely anonymous to defeat the value extraction AI aliexpress uses to show you higher margin sellers first.
With containers you can create one container to always open that website in it. But you can't choose which of all these context, you just get one default container per website. (or you have to manually manage them and it's easy to slip up and get tracked)
With multiple profile, you can name them and colour the firefox UI theme to represent which type of account this is. Anonymous/logged main/logged alt/ etc...
The problem is the profile switcher addon doesn't come as part of firefox base installation, you have to use about:profiles and it's terrible by comparison.
Profile switcher should be part of the base installation.