I sincerely suggest using vertical tabs! There's two popular FF extensions for it, the original Tree Style Tab [1] and Sidebery [2]. Most widescreen displays are far wider than web content these days, so you don't lose any horizontal space by switching to vertical tabs, and you gain the ability to actually read the text + organize them by hierarchy.

1: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

2: https://addons.mozilla.org/en-US/firefox/addon/sidebery/