Alternatively, if you use the LibRedirect Firefox addon, you can simply go to the Twitter section, click Ping instances, and only add instances that come back as healthy as favorite instances.
Currently, Nitter uses guest accounts to access content on Twitter. Guest accounts expire after 30 days (IIRC) of them being created. Guest account creation has been shut off, so all existing guest accounts will be dead after 30 days max. Nitter instances with working guest accounts are not dead yet, but they will be.
Nitter still works somewhat if you only use instances that are listed as healthy at https://github.com/zedeus/nitter/wiki/Instances#public.
Alternatively, if you use the LibRedirect Firefox addon, you can simply go to the Twitter section, click Ping instances, and only add instances that come back as healthy as favorite instances.
According to the comments they'll only work for up to the next 30 days