I'd be interested in purchasing themes for Firefox both as a way to sustain the development of Firefox and as an opportunity to get high-quality themes for the browser from a trustworthy source—especially considering the recent focus on Colorways.
One of the things I like most about Firefox is its potential for customization compared to other browsers. I regularly use userChrome and userContent to improve my browsing experience, the latter for removing unnecessary HTML elements for certain websites.
Mozilla Corporation could also sell high quality New Tab page templates. Not being a great web designer myself, what I came up with is serviceable but not visually interesting.