That doesn't sound very reassuring. Especially considering that uBlock Origin is one of the most important addons. So much for parity with Chrome as far as WebExtensions go I guess...
As long as you're here (I never wanted to open a ticket on Github, because it's something to do with filtering): Are you aware of issues with 'uneven' page loading with the WebExtension of uBlock Origin in Firefox? It only happens when cosmetic filtering is turned on. This is how it looks: https://abload.de/img/forumvzqc1.gif