Any feedback, suggestion, bug reports, problems....
At the time of writing this, the beta channel has a version equivalent to release. (the release candidate for the current version) I plan to test out caching improvements on beta, as I work to finish that bit (needed for the new features). I avoid posting betas that are known to be unreliable, and I don't remember the last time I had a public alpha test (I think I did that once, but I forget).
The new mandatory-to-use WebExtension API is intentionally limited, in part to limit the level of breakage addons can do, so I wouldn't worry too much about using beta versions from long-existing addons. (you might want to be wary of using betas from new addons, as the beta channel is not AMO reviewed and is unsigned) If I do somehow manage to set something on fire, please report it and feel free to go back to the main release, if needed. That said, the beta test period for the major release is over, so I will hopefully not be making any really big changes anytime soon. It is always nonetheless called the "Beta Channel" by Mozilla, regardless of if it has alphas, betas, RCs, minor update prereleases, etc.
(Apologies for the late reply.)