Just a heads up, but we are actively working on improving the situation here, and should have releases coming out that steadily and progressively improves the experience on this front. It is an area of major focus after the initial Quantum Flow effort, and we have technical fixes in many places in the pipeline to address this.

(I work on the Javascript engine in Firefox, and improving our story here is one of my personal top priorities, as well as an organizational priority).

Read in another HN comment just the other day that the slowness was because they used some obsolete only-in-chrome shadow DOM API which "polyfilled" on all other browser, which if true sounds exactly like pulling a Microsoft.

I haven't verified it myself and I couldn't find a reference with a quick search, so while it sounds plausible, it would be great to havd a credible source to refer to.

I have a Gmail account since it was invite only and I believe also there wasn't any web interface yet so I always used it through a mail client such as Sylpheed, now Claws Mail (https://www.claws-mail.org/). The very few times i had to use the web interface (always Firefox or derivatives) it worked without hitches on FF too, but a few months ago I was doing some maintenance to a customer PC (Debian+Firefox just like all my machines) and noticed how slow the Gmail web interface had become. Switching to the "old" mode helped somewhat to restore some speed, but being used to snappy dedicated mail client it still was truly slow to operate. Unfortunately today every non tech savy user thinks the browser is the internet to the point some don't even know what the word browser mean, which makes even more difficult to convince them to try anything else. Where a browser struggles with two pages of mails, I can keep available my entire archive since about 2001 on the Claws Mail client, that's over 50K mails in a dozen accounts, and query them to find whatever I need.