You said Firefox but we talk about Thunderbird here. :-) If you compose in Thunderbird using HTML, you can use '''Insert ''' on the compose window menu to find âçcéntèd letters and a few other frequently-used characters, including °. An alternative and more powerful approach is to install the abcTajpu add-on which allows you to compose characters, or select them from a virtual keyboard or from some rather long drop-down lists. You can also create macros for any that you regularly need. This add-on unleashes the full gamut of unicode and doesn't demand that you use HTML, and it's available in both Firefox and Thunderbird.