This is a copy from https://support.mozilla.org/en-US/questions/1210155 as per suggestion:
I’m looking for an addon that would display total downloaded page size (including images, ideally with the count of elements). Something along the lines of https://addons.mozilla.org/en-US/firefox/addon/extended-statusbar/ or https://addons.mozilla.org/en-US/firefox/addon/viewstate-size/ but working with Firefox Quantum.
Vivaldi has this built-in and it’s awesome feature!
(I know that I can see that information in web-developer toolbar but I want to have it visible all the time…)