From what I read, 79% of Nightly channel users and 7% of Release channel users on Windows are already running 64-bit Firefox. (May be more now; those numbers are from a May 2017 Bugzilla comment.)
And that will become a much bigger problem for LastPass soon.
As of August 8, 2017, the stub installers automatically install the 64-bit Windows version of Firefox when downloaded to a 64-bit Windows system.
Come October, the proposal is to migrate Firefox Release 56 users that have a 64-bit windows with more than 2GB real memory to the 64-bit build of Firefox with a 56.0.1 upgrade. (At least 2GB? More than 2GB? There has been discussions about that in one of the Bugs. Also, there are opt-out possibilities by setting a registry value, or by uninstalling Firefox and reinstalling a platform/language/build-specific version.)
See Mozilla Wiki: Firefox/Win64 for more details.
So, by the time Firefox 56.0.1 lands in the Release Channel, LastPass will have lots of 64-bit users. If they don’t support 64-bit users then, they may find their users looking for alternatives.