Because Mozilla (unlike most other web browser publishers) publishes free software -- software users are allowed to run, inspect, modify, and share at any time for any reason. This is known as respecting a user's software freedom. Programs that don't respect a user's software freedom are called non-free or proprietary.
That's the saving grace of Firefox. That's what makes Firefox better than any of the other currently widely-used browsers. A program's technical problems can be fixed, a program's speed can be improved (Mozilla recently proved this in the most recent versions of Firefox), but software freedom cannot be added to a proprietary program.
Therefore if you don't like how Mozilla treats add-on programmers or Firefox users, you have the software freedom needed to make a free derivative of Firefox which behaves in a better way (where "better" is up to you to define, it's purposefully vague). Perhaps you want your Firefox derivative to let its users easily install whatever add-on they like that will persist across restarts, or get add-on updates without hassles by checking multiple sources for updates, or installing add-ons signed with unfamiliar keys after getting a user's approval. I'm sure there are plenty of other ideas you could come up with to implement.
But with software freedom the limits of what you can implement are limits you impose on yourself.
This doesn't make Mozilla or Firefox evil and distributing free software is respectful of the users. Don't confuse unrewarded labor (complaining that Mozilla doesn't make Firefox do what you want) with respect for the users.
The Dissent add-on, similarly, is really just another single point of censorship forum. We can't evaluate whether it is better at respecting user's free speech until they are challenged in a serious way (such as the implicit threats to big social media firms when they are given vague inactionable requests to 'do better' when their CEOs are brought before Congressional hearings and told to respond favorably to Russiagate lies). Already on this forum we see some posters confusing freedom of speech with reading something that echoes their views. One post, for instance, claimed "[Dissent's] audience is primarily folk on the far/alt-right". As George Orwell said, "Freedom is the right to tell people what they don't want to hear". Or as Noam Chomsky reminds us, "Goebbels was in favor of free speech for views he liked. So was Stalin. If you're really in favor of free speech, then you're in favor of freedom of speech for precisely the views you despise. Otherwise, you're not in favor of free speech.".
So both Firefox's add-on repo and Dissent's forum are singletons that don't liberate users so much as they establish their respective admins as censors. Perhaps you could come up with a way to let users more easily distribute comments from their own comment database so that all comment threads on a website come from multiple servers, and no censor power exists because no single user has admin control over all of the comment servers. Dissent is also free software (licensed under Apache License 2.0) so even if you believe Gab or Dissent aren't to be trusted, you could choose to improve Dissent to make it decentralized.