Greasy Fork

Win7 SP1 | Waterfox Classic 2020.9 x64 | tampermonkey beta

This script doesn't take effect.
Now viewing any instagramer's mainpage will redirect to the url form like "https://www.instagram.com/accounts/login/?next=/…" that always need login.

I'll have to try with waterfox. I know that there are differences between browsers.

Do you see any error message from the userscript in the javascript console?

I've installed Waterfox Classic 2020.9 with both tampermonkey and tampermonkey beta.
The userscript works flawlessly.

Note that "stories" and "tagged" can't be unlocked. Instagram doesn't send anything if you're not logged in. I don't know how they are named in the interface in your language but these links are the ones just above and below the horizontal line at the bottom of the page header.
For the rest I don't see log-in dialogs anymore (unless clicking "tagged"/"stories").