Why can't I get Channel 5 subtitles with Firefox?

I contacted C5 asking why I couldn't get subtitles while watching their streaming or on-demand programmes. They said, after asking about my laptop, that subtitles on the website via Firefox aren't currently available. Why is this and is there a way around it? They did say that dependent on which programme, the subtitles will likely be available on the 5 Stream app, but I don't watch on the mobile phone.