Thanks for the reply. I was going to respond and explain that I tried all that (because I had) but I thought I'd try it again before responding. Strangely enough, the option to sign in actually appeared today in a space that it simply did not exist yesterday.
I tried v81, worked ok for a while and then the same issues. Extremely slow and laggy and unworkable with a lot of tabs even if most are being snoozed. I guess I have to go back to v77 or stick it out to see if any newer versions fixes this issue.
I tried them but the extension of user string not working on unacademy u can try too.
I am not able to create an account there, so can't try. But, you can try some other user agent extensions. Or, you could use opera://flags/#reduce-user-agent and use one of the extensions for opera.com to force identifying as Opera. Then, that should make the addons site work.