So Kestrel betas are flying out and high on the list of changes is a big, black important bullet point saying "improved site compatibility".
So, I can hear you asking: why is Yahoo mail
beta useless in the Kestrel preview? In fact, it is usable in 9.23 but completely broken in 9.5 alpha..
Patience, please. Precisely because some of the new features like getters and setters that are meant to improve compatibility when we finish them are work in progress, Y!Mail is currently rather broken. It won't take us many weekly builds to get things sorted.
There are two big-ish issues, one is that getters and setters don't work when defined on DOM prototype objects and the other is Mozilla's bug 45566 which we've "fixed" in Opera and it turns out Y!Mail really REALLY prefers the broken Firefox implementation. I have a feeling they'll never be able to fix that bug, they'll have to label it a feature in the end.. so we had better be
bugfeature compatible. 🙄 Naturally, DOMParser is their non-standardised extension so they can do what they want and we have to reverse engineer and obey.