lallmon Solus targets desktop computer users and more specifically the home computing segment (source: home page of the official site). 😁
Regarding Firefox Developer Edition, I agree with you that it's kinda strange to have Vivaldi stable & snapshot or Chrome stabe/beta/dev but only Firefox stable while it got a different engine, but Chrome for example is a binary package and it offered through the 3rd party repository which means no need to have frequent long builds and it can be updated independently from the ongoing activities on the main repository.
Unlike what you say, using the tarball isn't straightforward with basically everything (but it is with firefox), otherwise why would people work on universal packages solutions like flatpak, snap or appimage for example ?
Also a developer probably know how to extract & run a software from a tarball while this might be complex thing for a casual user, so the question to ask is should this be a priority ?
Manjaro relies on Arch which has a huge community. Solus should control its growth as it has a relatively small team and not so many contributors. What would be the point of growing fast and not being able to keep things up-to-date?
Now if you don't want to work with the tarball because it's annoying to download & update manually all the time, snap packages are available and also (unofficial) flatpak packages provided by Fedora/RedHat maintainers. Perhaps this could be a better alternative for you?
