This isn't so much a question, but more of a report to bring the issue to light. It doesn't appear to be very common, but I was able to resolve the initial problem with help from the SteamVR and Firefox subreddits.

I had an issue that only started happening over the past few weeks. I have SteamVR installed on my PC, and I'm using Firefox 55.0b3 which has the new VR components in it. However, I encountered issues where some part of SteamVR automatically launches every time I open Firefox (whenever my PC is on). This in turn causes GPU usage to fly up to over 80% - which draws way more power and produces more heat, as it normally sits at <10%. I've tried tweaking SteamVR settings, though it didn't make a difference despite various suggestions, so I then went into about:config to set dom.vr.openvr.enabled to false, which fixed the issue. SteamVR no longer opened non-stop (re-opens after closing it or ending the process), GPU usage went back down to <5%, and Firefox's memory usage also dropped from 1.5GB to 500MB. This may well be a bug with SteamVR as well, but it only occurs when Firefox is open.