That could indicate a problem with the BT test, an issue with Firefox, or a problem with the BT service.

The test I see http://diagnostics.bt.com/login/?workflow=Speed appears to be for BT customers & I am not a BT customer so I can not try to reproduce this issue.

What is more important is whether you are having problems because of your Internet connection.

It seems you are not having problems and report:

By the way all other speedtesters eg Speedtest.net 
and my ISP's own all report about the same speed (69mbps) 
no matter which browser I use.

A further update to my initial post.

I have changed the router, tried the tests on 2 other computers - one a Mac running El Capitan (also the latest version like the iMac) and the other a PC running Windows 10.

I have tried connecting via ethernet and wirelessly and via home-plugs.

So all the bases have been covered.

The router was changed today to a new Plusnet Router received a few weeks ago as my original router was getting flaky and I wanted to eliminate the old router as being the problem. Connecting via ethernet or home plugs made no difference but wireless speed increased to effectively match the wired connection so the router change was worthwhile in itself.

After the router change the BTW speedtest via the iMac still reported the slower 27 mbps speeds when using Firefox but about the same 64 to 69mbps when using Safari or Chrome via a wired connection - ie the same as before the router change however the other Mac (a macbook pro) and the PC gave the higher speeds when using Firefox ie the same as Safari & Chrome.

The only difference between the Mac computers was that the iMac had the latest Flash Player installed (20.0.0.306) whereas the Macbook Pro , and also the PC, had Flash 20.0.0.286.

Suspecting this had something to do with it I updated Flash on the Macbook Pro and the PC and sure enough the download speeds reported by the BTW speedtest when using Firefox were reduced to the same as the iMac.

My conclusion is that there it is the combination of the latest Firefox and the latest Flash Player that is producing the anomaly. BUT WHY??

Please, has anyone any ideas, and any suggestions as to what I should do about this.

It may help to have links for the speed testers you are using if they are available to the public.

A speed tester is going to download or stream data, try to process that, and then try to give some result. The speed tester may well use Flashplayer, but that is not the only method available. Obviously if a speedtester uses Flashplayer and the browser or computer has problems with Flashplayer then you will get misleading results.

There are many other factors that may affect speed and the results. The ISP will have some form of throttling. Even if that is only due to the networks ability to share bandwidth. (May not be net neutral. May involve packet sniffing)

In this forum all we need to know about and try to help with is end user problems with Firefox. Detailed discussion of internet speeds is outside the scope of this forum.

Does an test via HTML5 work better?

• http://speedof.me/

You can check for problems with current Shockwave Flash plugin versions and hardware acceleration in Firefox and try this:

• make sure you have the latest Flash version:
  http://www.adobe.com/special/products/flashplayer/fp_distribution3.html
• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Windows Vista and later)
  https://support.mozilla.org/kb/adobe-flash-protected-mode-firefox
• disable hardware acceleration in the Flash plugin
  https://helpx.adobe.com/flash-player/kb/video-playback-issues.html
• disable hardware acceleration in Firefox
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Reply to John99

I use the following

http://speedtest.btwholesale.com/

http://speedtest.net/

http://www.mybroadbandspeed.co.uk/

The last one is the one linked into my ISP website (Plusnet)

I also use the latest Thnkbroadband speed test but only when browsing with Chrome as it doesn't work well with Safari or Firefox as their site explains.

I have reported the problem to my ISP who will investigate and ask BT (British Telecoms) to investigate to see if their is a problem with their tester not being updated to work with latest Flash Player & Firefox. I await their response but it will take a few days.

In meantime speed seems zippy enough to be consistent with the readings from the non BT testers and from the BT tester when used via Safari.

Reply to Cor-el

Speed of me gives the same result as all the others except or BT under Firefox which gives the anomalous low download speed.

All the other suggestion you made have be dealt with where applicable for a fully updated iMac running latest OS , latest Firefox and latest Flash.

So the conundrum remains for the present.

Adobe builds a slightly different Flash player for each browser, so it could be something in that particular version of the Flash plugin. Unless you are looking to do a lot of uploading and downloading through Flash, it might be more of a curiosity than an issue.

Who knows the next update to Flash might cure the problem. If it does I'll report back.