Using Jonathan Kamens' "Datepicker Bug Reproducer" add-on ported to legacy WE from bug 1492522, attachment 9010404 [details], type this into the error console: openDialog("chrome://dtpr/content/dialog.xul", "_blank", "chrome,titlebar,modal,resizable", null); Use the pulldown on the first date picker and then the < > arrows to move the month forward or backward. Result: The month name doesn't change. Jonathan, could you please take a look. We'll try not to punish any good deed this time ;-) (not that we did last time).

I'm not sure what you're asking me for here. Can you clarify? Also, I am using the daily build from March 21, which is 61.0a1, but despite the fact that I have the extensions.checkCompatibility.61.0 preference set to false, it won't let me install the referenced XPI, saying it's not compatible with my version of TB. Any ideas about that?

The XPI is your original add-on as a WebExtension compatible with TB 63+. Please download a TB 64 Daily and follow comment #0. It would be good if you could fix the date picker so the pull-down works again.

I will put this on my to-do list, but I do not know when I will be able to get a chance to look at it. It's still not at all clear to me that my add-on which uses the datepicker, Send Later is even going to be compatible with TB 63+ without me doing a great deal of work that I'm not sure I'm going to have time to do -- it seems like there's much that's still up in the air about what maintainers of legacy add-ons are going to have to do to make them compatible with the next released version of TB after 63? -- and frankly, if I'm not going to be able to keep supporting Send Later in TB 63+, I don't think I can justify spending my time fixing datepicker.

I understand. With luck, legacy XUL add-ons will "just work" with TB's home-cooked overlay loader. Of course only if they adapt to changed/changing interfaces, see https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_63#Removed_in_mozilla61 and below. I got "Signature Switch" going (not my own add-on), so that's positive.