To me, Firefox is an indespensible browser (partially) due to the hackable nature of userChrome and what can be done with extensions. I have long been a proponent of tree style tabs; hierarchical navigation makes much more sense for organizational purposes, and vertical presentations of browser tabs (with autohiding) gives more screen real estate when web browsing. Then you can close N tabs at once if you close a parent (and have your tree settings configured to close child nodes), which really enhances the user experience and boosts productivity.
Hence, why I've adored Firefox's Tree Style Tabs for many years. Tree Style Tabs has a new competitor on the block: Sidebery.
Sidebery provides tree style vertical tabs, with customizable panels. The panels leverage Firefox's concept of containers (a method for you to isolate session). So you can have various instances of trees in panels! And panels don't need to be limited to site tabs; you can have your panels dedicated to other things, like bookmarks.
This means that you can quickly switch between instances of trees (to isolate session, or to separate work flow (personal, business, media, etc)), bookmarks, and other panels/containers while also organizing and traversing your tab space via a tree. This is super powerful, and with userChrome, you can hack this even further to autohide the Sidebery, have each tab be a different color, etc.
When not hovering in the hover region:
When hovering in the hover region:
my bookmarks
my work panel
my personal panel
Link to the extension: https://addons.mozilla.org/en-US/firefox/addon/sidebery/