I recently started using Thunderbird on the Mac (after using Mail.app for a couple of years) and it’s great and all, but why doesn’t one of the delete keys work? On the Apple keyboard I have, there are two delete keys: one under F13 and one below the “help” button. I didn’t realize it until I started using Thunderbird, but I always use the latter key (the one below “help”) when I want to delete e-mail. That delete key doesn’t work in Thunderbird on the Mac. Other people have noticed — it’s not just me.
I’m one of the odd people who uses a Mac keyboard for his PC — I like the Apple keyboard and I switch between a Mac and PC via a KVM switch, so it makes sense to stick to a single keyboard. And guess what? The delete key on my Apple keyboard that doesn’t work on Thunderbird for the Mac actually works on Thunderbird for Windows.
Go figure.
Chad Dickerson's blog 
I just switched to mac and this the missing delete key in thunderbird drove me batty, but i found the following on a conspiracy site:
FN+Backspace and that works as a traditional ‘Delete’ key
on my windoze keyboard, the Shift+Backspace becomes Delete for Thunderbird’s purposes and deletes mail with 1-1/2 click (because so far apart)
Chad, download and install KeyConfig extension to Thunderbird. Then open its preferences using Tools->Keyconfig, click Add a new key, name it as key_delete, add “goDoCommand(‘cmd_delete’)” to the big box, check Global checkbox and click OK. Then select the newly created key, highlight the text box on the left of Apply button, push Delete key and finally click Apply and Close.
Now delete should work as backspace.
The FN+Delete works perfectly! I run Fusion on my MacBook Pro so I can check the apps/sites we work on on both platforms and it’s been driving me crazy to not have that Delete functionality without hooking up my PC keyboard. 2 years later and your post/solution is still relevant! Thanks:D
I echo Marlene in every syllable. Thanks, 5ymptom!
fn backspace doesn’t work on my imac!
help!
the more i think about not being able to deleate on keystroke the madder i get!
i only use the keyboard to do most of my work and i hate using the mouse because the time it takes you to do something with a mouse will always remain the same while keystrokes will always improve!
anyone got a soloution yet?