Those aren't good reasons from the user's perspective. Those reasons were good reasons from a technical, and maintenance perspective (which, as a software engineer, i understand). Those reasons are what engineers give to product management in order to get a green-field rewrite.
But firefox's value add is their addon system - even if it is old, and difficult to use. It enabled some really great addons that weren't possible in the new replacement (as a simple example, the DownThemAll was rewritten using this new api, and it's shadow of their former self).
Firefox should have paid the cost to maintain the old legacy system, and replace it only when the webextensions api is sufficient to replace _all_ functionality (which, of course, won't because webextensions evolved from google trying to kill off addons https://developer.chrome.com/docs/extensions/reference/ ).
Not to mention XULRunner/XPCOM was Electron before Electron. I remember using the Songbird app, as well as an IM client whose name I can't recall now, across Windows and Linux back in the 00s. So They literally decided to cede the cross-platform browser-based desktop software framework du jour to Chromium.
But firefox's value add is their addon system - even if it is old, and difficult to use. It enabled some really great addons that weren't possible in the new replacement (as a simple example, the DownThemAll was rewritten using this new api, and it's shadow of their former self).
Firefox should have paid the cost to maintain the old legacy system, and replace it only when the webextensions api is sufficient to replace _all_ functionality (which, of course, won't because webextensions evolved from google trying to kill off addons https://developer.chrome.com/docs/extensions/reference/ ).