I realized that the Firefox, Thunderbird, and SeaMonkey Portable launchers aren't fully portablizing paths within prefs.js. They portablize the drive letter, so you're good if you use it on an external drive or locally. But they were never updated to portablize partial paths so they'll work properly from a synced cloud folder between PCs or if you move the PortableApps directory. I'm working on an update rewrite to Firefox Portable's launcher now to address this and improv code readability for some of the code that hasn't been updated in 10 years and will then move it to Thunderbird and SeaMonkey as well. I'll post a test version as soon as it's available because I'd like it to get some testing before pushing it out to the official channels. Feel free to follow this bug to know when they're available.