I'm not sure how what you describe is not what a tiling WM offers. Except for the "closing main window" thing but can't you just cancel closing with the history menu?
Imagine an IDE that allows you to see multiple files in an X by Y grid - replace "files" with web pages. Is expecting that feature in a browser that far fetched? Or are we resigned to creating multiple windows + tiling WM as the one-true-solution for this usecase?
You're not describing how a tiling WM fails to satisfy your needs. My guess is you're unhappy with toolbars being repeated in each tile, is that it? If so, I believe Suckless's striped-down browser is the closest, together with a tiling WM.
If code editors are indicative, yes some power users tend to like a tiling window manager bundled in the app. So you may be right but I think it's out of scope for Firefox which targets the masses and it's definitely off-topic here.
> You're not describing how a tiling WM fails to satisfy your needs.
Yes I don't want repeated toolbars because they are waste of space when they are repeated in each window/pane - having to switch browsers isn't the point, the discussion is about what Firefox can do to improve its user's life (even if everyone doesn't expect/need split panes in their browser, some do). Because I don't want to manage 4 windows tiled like 4 panes - I want to manage a single window with 4 panes. What happens if I close one of 4? Now I have a gap that an related or unrelated window will fill or it'll be a wasted space - this doesn't happen in case of split panes because the other panes will expand to take the space. I logically group my work in separate windows - so in a split pane setup, once I'm done, I close a single window instead of closing the separate "pane-like" windows one by one.