Mozilla reviews and test every extension public available and approved, which is the case of all extensions in the package manager. So you are pretty safe using Firefox add-ons manager or even downloading from Mozilla add-ons site, as long as you don't download unapproved extensions.
Downloading from Mozilla has the benefit of getting the most up-to-date versions, which is particularly important for extensions that needs to be updated frequently. For instance I had to release a new version of one of my extensions 10 days ago, because it stopped working due changes on YouTube web site. Six days later, Vimeo changed their web site too and the extension stopped working again, so now I'm rewriting the extension. This doesn't happen very often, but when it does, you get stuck with a non-working extension, if you can't get an update right away. To be honest, I don't know how often extensions are updated in Ubuntu package manager, but I suppose not much, since they follow the same update policy for other Ubuntu applications.
The advantage of the package manager would be stability, but if something goes wrong with an extension update from Mozilla, you can always go to the extension page, add /versions to the url and download the last stable version.
I used to have more than 60 extensions installed in Firefox, and except for Ubufox, all of them were downloaded from Mozilla.
Bottom-line, in my personal opinion, you should get your extensions from Mozilla site.
BTW, add-ons is a generic term used by Mozilla to describe extensions, plugins and themes. But there is a big difference between "extensions" and "plugins". What I'm talking about here are extensions, since plugins are not hosted on Mozilla site. They have a page about plugins and the browser has a functionality to check for plugin versions, but the update process is done via developer site. The automatic plugin installation, that is presented when the browser detects you don't have the necessary plugin for a specific content, does not work on Ubuntu and the plugins must be installed manually or via package manager, which is the most common on Ubuntu.