Firefox was awesome, because of how much it could be modified by extensions / addons. With Electrolysis (aka e10s), they are doing away with their XUL foundation for extensions, and the new foundation does not afford as much customization of the browser. Existing extensions, of which there were over 12,000, unless updated, will become incompatible. I think they are killing off the uniqueness of Firefox, that maintains its existing user base.
Why would they do this, intentionally? Because they don't like to share control: “The tight coupling between the browser and its add-ons also creates shorter-term problems for Firefox development. It’s not uncommon for Firefox development to be delayed because of broken add-ons.” More importantly, they have conceded to Chrome as the winner, and are adopting a format for extensions that is compatible with existing Chrome extensions.
Pale Moon is a fork of Firefox, that is lightweight on system resources, and works with a majority of the XUL based extensions available for Firefox. Pale moon will not be adopting e10s. However, since the user base is a minority, the garden of extensions is likely to continue to wither away.
At the end of 2012, I tried to configure Google Chrome the way I had Firefox, since at the time, they had a plentiful library of extensions. I couldn't get what I wanted, because their foundation for extensions does not go as deep into the workings of Chrome. Perhaps I am exaggerating. I got 80 to 90 percent of what I wanted. I did notice, though, that Chrome consumed more memory than my Firefox / Pale Moon counterparts.
I can continue to use an old version of Pale Moon for now, but going forward, I may look into the Vivaldi Web Browser. It is based on Chromium, which is the code that Google Chrome works off of. Chromium and Chrome are not very different.
I think the best design for a web browser, is to be minimalistic, where you can select more functionality through extensions. However, the features added by Vivaldi seem to be few and well selected. The head developer of Vivaldi, used to work for the Opera Web Browser. I never liked how Opera included a lot of functionality I either didn't want, or preferred from another source. I hope he keeps it simple.
While I prefer lightweight web browsers, I don't use them as my primary browser, because they are not mainstream enough to have a rich library of extensions.
I really don't want to use a Microsoft product (it would be like voting for Trump). Microsoft Edge has a dismal library of extensions. There are 5. Internet Explorer didn't have many either. Perhaps there isn't much interest, from anyone with intelligence enough to code an extension.