Tree Style Tabs was the only blocker for me, and I was amazed that it was possible to recreate under the new API.
I use both Firefox and Chrome, but for very different purposes. In Chrome, I have 20-50 tabs spread across two windows. In Firefox, 400+. Tree Style Tabs is necessary for how I use Firefox. My backup plan was staying on an older version, possibly indefinitely.
The APIs are still being expanded. Tree-style tabs was near the top of the list of things they wanted to get support for before the deadline. They also had a bunch of bugs open for various features where devs could discuss use cases for potential APIs, and even "office hours" to work directly with extension devs to help them port their add-ons.
I was also very, very concerned about Tree Style Tabs. It's one of the main differentiators Firefox has for me over Chrome. Very happy it's mostly been preserved. I really only care about having my tabs on the side, not necessarily the nesting.
I personally find the nesting excellent because I am a junkie that opens 90 tabs at a time meandering through the internet and it gives a way to trace my context/history.
You're probably better off not updating yet. I'm unable to open my old session after the 57.0 update.
1283 tabs open in the session that I'm trying to open.
FF56.0 the session opens in around 20 seconds.
FF57.0 I let it try to open the session for 40 minutes and still wasn't finished.
Not sure how it would perform if it would manage to open all the tabs but currently it's pretty much unusable for tab-heavy user.
When starting a new profile and starting session from scratch the browser seems really nice. I'll try nightly at some point and see if it's any better. If not I guess I'll write some tickets for them.
I'm sorry if I sound a little condescending, but have you considered using bookmarks instead? I cannot even imagine a workflow where four flippin' hundred active tabs are needed.
It's not one workflow, and there is no scenario in which all 400 tabs are active at once.
Bookmarks take an extra step to save to, an extra step to load from, and do not stay in sync as I browse. Synchronizing a bookmarks folder with 10-20 tab changes would take significant human overhead.
Bookmarks are also slower to review than tabs, if you need to see anything besides the name/url/icon. You would need to load the bookmark into a tab before viewing it. Tabs are already in a tab, though not necessarily loaded.
You can be almost certain that people using hundreds of tabs have considered bookmarks and didn't find a better workflow using them, yes.
(Personally, I believe a good bookmark-like system could solve most reasons I have many tabs. But I neither know what exactly that'd look like nor do I want to spend the time developing it, so tabs it is)
Bookmarks have a really, really bad interface compared to tab trees.
Bookmarks are flat by default. You can make folders, but that takes manually opening the bookmarks manager and placing newly created bookmarks into the appropriate folder. Folders also waste space in the tree; one bookmark can't directly be the parent of another. The web doesn't have folders, it has links.
Bookmarks don't preserve structural context the way tab trees do. For example when using an API documentation site I'll open the site in a window, and from there open classes I need info on in tabs. Methods or related classes go in sub-tabs. I eventually end up with tabs for all the bits of the API I need to reference AND THE STRUCTURE!
Bookmarks are also hidden behind a menu, and are slow to access. Tabs can simply be suspended to save resources, and their trees collapsed.
Etc, etc. Bookmarks have horrible UX compared to tab trees.
Trees collapse, and typically represent recursive exploration of some particular area, and can act as a kind of task list or reading list. You collapse the tree when you're not actively drilling into that topic.
Combine with a solid session manager (I use Session Manager) to back them up regularly, and they fill in a third space between an open tab and a bookmark: something you only want to visit once, some time in the next few days / weeks, and have no desire to keep around longer than that.
Maybe it is just me, but Tree Style Tabs is ridiculously slow on FF57. Takes about 3 seconds between the moment I hit F1 and the moment the TST sidebar is done loading.
I use both Firefox and Chrome, but for very different purposes. In Chrome, I have 20-50 tabs spread across two windows. In Firefox, 400+. Tree Style Tabs is necessary for how I use Firefox. My backup plan was staying on an older version, possibly indefinitely.