What functionality are you missing? All the old menu's are available if you really want them as far as I am aware, just hidden be default.
That's a big downside for me but what can I use instead of Thunderbird? I worked around that issue by sticking some folders to the top of the list using Favorites. It's 1% of MSF functionality but at least I finally used Favorites.
[1] https://github.com/protz/Manually-Sort-Folders/issues/199
Global Inbox also got broken several years ago and never fixed, since it seems all their energy goes into making things prettier at the expense of making things work.
And even that is a matter of taste. They tend to make things resemble some combination of an Android app aesthetic and a Google Chrome aesthetic. For example, replacing dialogs with item hierarchy trees with a scrollable flat web-form-like preferences page.
At least they backed out of the switch to monochrome folder icons. 8-\
2. No multi-folder selection. This is necessary for actions involving multiple folders - from a simple move to extension-enabled actions like "Search these folders for duplicate messages")