Two days ago Firefox broke all add-ons because they forgot to renew certificate. This includes downloading add-ons.
There is now a lot of articles about it over the internet, the official tread where the issue is being tracked and solved is here:
But more info about current state is on their Twitter page:
Right now, the fix they released should be working for most users - just keep your Firefox running for few minutes / hours and it should get automatically fixed.
Also you may need to restart Firefox and then wait again few minutes to re-enable addons.
Alternatively, you can check for updates - there is Firefox 66.0 4 version on the way that solves this problem as well.
Another option is temporary load your addons, see this short video I've made: