Either way, I just wanted to note you can use APT to include Mozilla's Firefox and Thunderbird repo. That way, you get a .deb from them.
One specific thing I like about Ubuntu: I know a new version gets released every six months (I believe OpenBSD innovated this?) and the version is recognized by date. The LTS is released every two years around april in the even years. Otherwise it is april/october.
Since you're switching to Debian, could you tell me what you don't understand in Debian that was working fine on Ubuntu for you?
I am not an Ubuntu user, so it's hard for me to know. But I find it very surprising that after 10 years on Ubuntu, it would be "annoying" to switch to Debian.