This extension allows you to spoof your browser "user-agent" string to a custom designation, making it impossible for websites to know specific details about your browsing arrangement.

You can easily set custom "user-agent" strings from the toolbar popup. The extension uses a two-factor technique to mimic your default "user-agent" string, which is the most reliable method. The extension does not use any resources when it is not spoofing the user-agent. The extension enables user-agent spoofing on specific domains only; it is conversely possible to exclude certain domains from spoofing. Check the Options page for more info. You can set per-site user-agent strings. There is an option to randomize user-agent strings. It is possible to set user-agent string for a single window; you can have a separate user-agent string for each browser window.

Documentation: https://webextension.org/listing/useragent-switcher.html

How to configure nginx to block certain user agents from accessing a site.