The new Opera is very, very simple, but it allows you to install just about all extensions in the Chrome store, if you download the Opera extension to do so.

So it depends on which features you'd prefer to keep.

I can definitely recommend using Pinboard for your bookmarks anyway.

Firefox, Chromium.

I think Mozilla may be headed in the same direction.

Add-on File Registration System: http://www.ghacks.net/2013/11/01/mozillas-add-file-registrat...

Merging of AMO with Firefox Marketplace: http://www.ghacks.net/2013/10/26/thunderbird-seamonkey-kicke...

In Firefox's case, at least, it's clear that the only motive in doing so would be genuine concern for the safety of their users— Mozilla wouldn't have any incentive to stop people installing extensions that hurt their own business model.

Firefox Marketplace is open source. It's based on Django and you can host your own. Source: https://github.com/mozilla/zamboni

Doubtful. Mozilla has repeatedly gone out of their way to ensure that their services aren't strictly reliant on a centralized source. Firefox Sync allows you to set up your own sync server. Persona allows you to set up your own authentication server. Firefox OS allows you to set up your own first-class app store. Locking down Firefox add-ons in the same manner as Chrome would be highly out-of-character.