As an addition to the translation tool (context menu in Firefox) it would be nice to have a conversion tool for the conversion of temperatures and lengths. I know there are addons for this purpose, but last time i checked they weren’t good or they were a hassle to use. Opera nailed it a few years ago with a little pop-up window when text got highlighted. It recognized when it was a SI unit. This is a feature that I have not seen in any other browser yet.