Can I download certified/verified add-ons for FireFox and if so where can I download them from?
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is there a source of known-to-be-portable addons, then please use the site search system. This has been discussed many times before.
There are literally thousands of addons for Firefox, and only a tiny handful that have any problems with portability. There is no way that this site could feasibly maintain an up to date list of the addons, let alone test every new version of every addon to certify it.
Most addons work fine portably, and leave nothing behind. The exceptions are
but 99% of the addons on addons.mozilla.org do not fall into any of those groups.