Configuring it is super counter intuitive. I have a paid hosting on app.wallabag.it and yet setting up the addon needs to create a client on the website, pasting it back on the addons settings page and still entering the password anyways, or maybe not, very unclear. I then realized that before setting it up it was not saving anything. So I just LOST all the pages I bookmarked before figuring out how to connect the plugin to my account. And the wallabag plugin doesn't indicate in anyways that it is linked to a server or not. No color change, no error messages. So I was just casually bookmarking pages with the idea that they were getting saved....But they are gone. Now that I linked to the server, I was hopping that it would sync a kind a local buffer, but no luck with that.. Wallabag is great, but the Firefox addon is just crap.

Works fine if you manage to get through all the hops for initial configuration (Firefox 58.0.1). It didn't picked up my url and didn't showed the rest of the settings (api id, secret etc.) until I restarted Firefox and after I filled the rest of the authentication info it reported the token as expired until the next Firefox restart.
Once it was working I exported the addon configuration (thanks for adding that option) I realized that it is storing passwords in plain text, please, never do this! To be fair, I am not sure if there is support for this in current Firefox api.

But anyway, thanks a lot for providing this addon!

Edit 12.02.2018:
Came back to take back one more start as this addon sends every url you have visited to the wallabag instance you are using (namely to yourwallabag/api/entries/exists.json) even if you never touched the wallabagger button, I find this quite a bad practice.