Not so much a 'why' but it does have a good history of the user-hostile moves Mozilla has made. https://digdeeper.club/articles/mozilla.xhtml
Off-topic; it’s great to see a good old-fashioned XHTML web page – with a Strict doctype to boot! I remember hand-crafting websites using XHTML (at first Transitional, then Strict) using Server Side Includes as the only backend technology and being proud at how semantic and human-readable the source code was. The markup on this page is elegant, clear and readable while the rendered XHTML is both accessible and responsive.