Firefox

Customizability, and most importantly portable customizability, is non-existent in most modern software. Even highly configurable FOSS products like Firefox take time, effort, and above average technical skills to mirror configs between not only clients, but identical clients, and even just keeping them in sync. You shouldn't have to log into their cloud or jump through hoops; just export a config file(s) and import it onto another machine; if versions are identical, the state of the product and its feature-set should be identical (e.g. not temp, session, or 3rd party data; though in the case of Firefox all extension configs should apply too, as they are part of its ecosystem/feature-set).

As a consumer, I often wish that there were a requirement for ALL digital product software and services to export/import their current config, including all "user" data, to open-source compatible, lossless formats — this would be the most effective method to free consumers and businesses alike from vendor lock-in and monopolization — but as a developer I'm aware that would be a nightmare, especially for all pre-existing software.