That's just how the internet works and what should be expectect when streaming video and downloading multiple files, there is less bandwidth to share, when closing the video stream you free'd up extra bandwidth for the downloads to use more of the available bandwidth allotment. unless you have an extention that throttles or eurosport has raised the bitrate of their video could cause aa difference, even time of day makes a difference.

The small change in the graph could be somthing like a change of Codec the website is using for video but I'm leaning towards a higher quality of video they are now delivering. It is possible Firefox is now asking for a higher quality of video by default

In your "about:support" typed in the url bar does it say D3D 11 beside Compositing? Also update your video card drivers directly from intel/AMD/nvidia

Have you looked at the graphs though? They show the bandwidth taken by all incoming traffic. In the second one the average speed is 7MB/s, in the first one the average speed is 4.6MB/s (I did the math). The difference is obvious, the first graph never even reaches the steady max value of the second graph.

That's not a small change, and I don't think that's the expected behavior.

I tried changing the performance settings, that didn't make any difference. I can open a second Eurosport video, both play fine, so it seems like the PC performance shouldn't be the bottleneck here.

Yah that seems perfectly normal bandwidth differation, different video feeds carry different bitrates (the amount of data used to show the video). Some feeds are 23fps, some are 30fps, some are 60fps so just the framerate difference can cause that. Youtube for instance usually uses either H.264 or VP9 video. H.264 will use more bandwidth in general (but less work for your computer to decode it) where VP9 will take a good portion less with a bigger hit on your CPU.

This is not taking into consideration of resolution, 360p, 480p, 720p, 1080p will all carry a huge difference.

Same Video Stream: H.264, 30fps, 360p @20mins would be 60MB H.264, 30fps, 720p @20mins would be 128MB VP9, 30fps, 720p @20mins would be 114MB H.264, 60fps, 720p @20mins would be 256MB

So you're looking at more then double the bandwidth just for the increase of the resolution or an increase of the framerate. Of coarse I can't 100% rule out anything funky going on, but wide margins of difference like that are completly normal.

Those are from youtube where they could send you any one of those by default making a huge difference, other websites with video could be more unpredictable.

If you looking to reduce the bandwidth used, pick the lowest frame-rate, lowest resolution, lowest quality and pick VP9 as your codec. But very rarely would you ever get that much control. Firefox or the video players performance have no effect on the data transferred, that includes using hardware acceleration of not.

You might be able to find an addon that lets you tweak it the video streams a bit, but don't count on it too much...

Things that could have changed: Before you updated Firefox, it's possible it was using the Flash video player to play back the video's on that website and now it's using HTML5 which allows you to recieve better quality video, thus increasing the data being tranferred. Not much you can do about that, Flash is outdated and being retired across the web as obsolete. Also possible you had an extention which requested a certain quality of video from the website and finally the browser will benchmark itself to let the website know what kind of video it can handle, since Firefox is faster now it's possible you got bumped up to higher framerate (but this is a real longshot).

Also many websites adjust the quality of the video. dependant on how fast your internet connection is, so one day you may get a higher bitrate/quality of video due to less network congestion from your internet connection (if you use Netflix you will notice this).

So there that pretty much convers a mini lesson in streaming video :P

You can download better network monitoring tools like WIreshark to be sure you haven't been comprimised, run traceroutes on where the data is going and be sure, if possible to be hooked up via ethernet cable and not Wifi for a better connection.

The situation is like this: I have many downloads going, they take all available bandwidth. I open the video, and now the incoming traffic (the same downloads plus the video) does not take all available bandwidth, it takes only 66% of it. That is the issue. Everything else is beside the point.

To your point bandwidth is determined by the Browser and O/S and router which one will allocated bandwidth to what the preferences is. And any available bandwidth will be taken by the first available programs unless the user sets the limits in the Browser or site and O/S to limit what take priority on the bandwidth. If left default they will run by default and also what type of ISP connection you have Fiber would do better for Streaming as opposed to Cable which is shared and depending on Time/Day can be fast or slow. So unless there is a software continuously monitoring bandwidth and what uses of the bandwidth this is all subjective as to what is hogging the bandwidth.

But that's backwards, the problem is exactly that the video prevents Firefox from using all available bandwidth. The downloads are happening in Firefox too.

Then your going to have to give a link so others can see if it happens when they watch it as well?

Unfortunately that happens with any video and any download source. As soon as there is any activity on a web page, the total bandwidth usage goes down, not up.

The quality of the video makes some difference: with a 480p YouTube video, the total incoming traffic speed (downloads plus video) occasionally reaches the maximum values that my connection allows, with a 1080p video it wobbles like shown in the graph.

http://www.speedtest.net/ Look in Device Manager under Network Adapters the 1st one listed should be the info you need, write the info down and search for New Drivers.

Note since running Win 8.1 note that you may already be at the max speed of that network card/chip. Can also Right Click the entry and have Windows look for a Update though if finds one does not mean the best one.

The internet/You ISP has passed your capabilities.

Please let us know if this solved your issue or if need further assistance.

No, it hasn't solved the issue.

When the downloads are happening in a separate app (a download manager), the monitoring tool shows that the traffic is at the max speed that my connection allows almost all the time, regardless of whether Firefox is playing a video or not (the monitoring tool shows all the traffic combined, the download manager plus Firefox).

While if the downloads are happening in Firefox, the speed is nowhere near the max network capabilities, as the graph shows.

So Firefox's download is currently unusable for me.