Starting July 11, 2023, Pocket users will be prompted to transition to a Firefox account in order to log in and access their Pocket account. Aside from changing how you log in to Pocket, this does not affect your saved items. While optional at first, the transition to a Firefox account in order to log in will be required by August 15, 2023.
For Pocket Premium subscribers, converting your account will not impact your subscription.
The addon really only spoofs the UA to a Chrome one, so Firefox gets the same page that Chrome does. In other words, this should also happen if you likewise change it with another addon.
The issue is likely that Google Search's Chrome version is now using some code that somehow hangs on Firefox, and I haven't been able to reproduce the issue to diagnose it, sadly.