Didn't work for me using Gmail with Firefox 55.0.2 running under Windows 10 (ugh!).

I'd been using Case Change Tool 1.0.rev44.1-signed.1-signed by Harry N without any hassles for many years but this does not work in the latest version of firefox.

I've now switched to TitleCase 3.1.1 by Joshua Pack. This does transform strings into Title Case, Proper Case, Start Case, Camel Case, Upper Case, and Lower Case. You have 2 ways to change text. Either by right clicking on the field and changing the case or by highlighting and only changing what you highlighted.