It's not so much the users they care about but the fact they have to deal with all the support requests, plus the complaining on social media that Firefox is dumb and bad. At some point it's net-negative for the project. So sometimes they open functionality to extensions instead of having it easily available to users, like the option to disable JavaScript on pages.

I should clarify that "users suddenly being without a tab bar" might not necessarily be the result of those users disabling it themselves. It might be that they install an extension like TST that doesn't restore the tab bar after it's removed, or something even less obvious.