All my plug-in's are up to date, as well as Firefox itself, yet I cannot upload images (my art) to multiple websites (tumblr, facebook, deviantart, etc.). Why?

In addition to the above, I've run security scans on my laptop, and have no viruses and the like. I've also disabled ad-blocker, and my other add-on's. As I said before, I am trying to upload my artwork, so there shouldn't be any sort of copyright issue, and I know which file types and file sizes I can and cannot upload. I'm running Windows 7 on a 2012 Toshiba, and just started having this issue last night. Also, this isn't only happening on Firefox, I've tried Chrome and IE too.