In Thunderbird I'd like the CTRL+R shortcut to do a REPLY TO ALL instead of just a REPLY. Is there a way to configure this that does not involve installing third-party extensions?
Keyconfig allows you to configure keyboard shortcuts.
Read more about it here.
Otherwise,You can unzip Thunderbird's chrome .jar files and edit the .dtd files within them, which usually contain the key bindings, if you want something that's not extension-dependent. Here's an example from a DTD file for my extension:
<!ENTITY removedupes.remove_duplicates_menuitems.remove.accesskey "D"> <!ENTITY removedupes.remove_duplicates_menuitems.set_originals.accesskey "o"> <!ENTITY key-removedupes.modifiers "alt,shift">
this is of course much less convenient than using KeyConfig, but that's the low-level non-dynamic way to do it.
It pretty much does all you can imagine, albeit with some shortcomings.