Thunderbird: Reply/Reply all: a couple of formatting questions

(A) Always use the same identity when responding I think that the only way to do this is with add-ons. There is a way to set a default outgoing account, but this default is overridden depending on the current folder selected.

An alternative is to use the Virtual Identity add-on. This is probably overkill, and I'm not sure if there is a slimmer alternative. I don't know how I'd set it up exactly, but you might be able to set up a wildcard "to:" address, and specify an associated default "from:" address. But I'm not sure.

(B) Suggest an add-on to format response header in particular way The "Change quote and reply format" add-on may do what you want. If not, there are also additional functions as stated on the linked page.

Again, I'm not 100% sure if this will help, but I thought it better to give you some possible leads.