That is the important issue. While Firefox can have 32/64 bit versions, there is no such a relevance to WebExtensions.
Unless there is an extension that deals with hardware (legacy ones could but WebExtensions cant), for the extension operation, there is no relevance whatsoever whether the operating system is 32 bit or 64.
Therefore, the assertion by a developer that a WebExtension (i.e. password type extension) does not have a 64 bit version, seems misplaced.
The only way a 64 bit OS may become relevant is that if the extension relies on an application that must be installed on the user computer and then the WebExtension uses Native Messaging to communicate with that application, then if they don’t have a 64 bit application, that may be an issue, although 32 bit applications run fine on 64 bit OS.