This is indistinguishable from the conspiracy theories people used to circulate about a magic carburetor design which got 85MPG and was killed by Detroit automakers for unknown reasons.

Just look at the chain of unsupported assertions which have to all come true for this to make any sense: Mozilla will prevent you from installing ad-blockers, and that this will bother enough users to matter but somehow that won't lead to enough volunteered developer time to maintain even an almost unmodified “fork” which changes only a build flag (or a signing key)?

Or that somehow if that proved popular enough to attract a large number of users they'd react not by reconsidering such policies but instead push everyone over to Edge/WebKit? Microsoft and Apple are not primarily advertising companies and at least Apple is marketing actively on the idea of respecting your privacy – it's hard to imagine anyone working at a browser vendor not realizing that such a move is simply going to push users to switch.