It seems that mainly due to security and speed aspects, then unlike legacy Firefox ESR 59.9 (with Tab Mix Plus and a few other extensions. which enabled more key customizations than the whole of Edge):
1. You cannot make it show multiple tab rows (you can with Firefox
Quantum but it takes one [1]of the hacks. And unless the latter is a portable version, it cannot run concurrently with FF ESR)
.2. You cannot adjust max or min tab width. And the more you open the thinner they get, and all look the same. (except the active tab has an close X)
3. You cannot close a non-active tab by just placing your cursor on it the top right
to get a X to click on (so unless you activate it, you need to R. click on it and hit close tab).
4. You cannot enable different colored tabs as with the legacy Colorful Tabs extension (NOT the one in Quantum).
5. You cannot choose to open links in other installed browsers via right click.
6. You cannot toggle btwn the most recently accessed tabs using the Ctrl+Tab switch (and its Alt+Tab alternative messes up switching btwn applications).
7. Edge has a very poor spell checker and the dictionary is hard to find in the drive even if you could edit it (as you can with that of Firefox).
8. You cannot create more than one profile (as far as i know).
9. I know no extension that saves your text box posts such as this, like the Lazarus legacy FF extension did/does.
10. MS does not make it easy to set another browser as the default one for all html documents and links (details too long for here).
11. There is no option to keep the Find/Search bar open across tabs (handy in searches), meaning you need to do the search new for every tab.
12. There is no option via right click on a tab to choose “Copy link.”
Since MS writes the software for Windows, than it ought to be able to create a real "utility truck" browser - and which I am sure would become a classic for "power users" - rather than just another minimalist Chrome clone.
Footnotes