Simple. Perfect. Rated 5 out of 5 stars
I like to read mail in text mode to protect myself from all sorts of known and unknown vulnerabilities. Every once in a while, though, I get an email from someone I trust that is unreadable unless I use "original HTML" mode. In theory this is no problem for vanilla Thunderbird, hit the "View" menu, select "Message body as", then select "Original HTML". Not so bad. But then I get involved reading the message and forget to go back to plain text mode, leaving myself vulnerable. And even if I do get it right, the round trip is 6 clicks.
"Allow HTML Temp" allows me to do exactly what I want in one click. If I trust the email, I simply hit the button and I'm reading it in HTML mode, and the next message I read is automatically read in my default mode (text in my case). Saving 5 clicks is good, saving me from forgetting to switch back is even better.