BTW Speedtest Results anomaly

[#1]

For the last two days I am consistently getting download speeds report as between 64 and 69 mbps using the British Telecoms Speedtest when using latest version of Safari or Chrome BUT only 27 mbps when using same speedtester with latest version of Firefox.
By the way all other speedtesters eg Speedtest.net, Think Broadband etc. and Plusnet's own all report about the same speed between 64 and 69 mbps) no matter which browser I use.
I am running a Mac on latest version of El Capitan. Latest Flash is installed.
Any explanations of help would be welcome.

[#2]

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.

[#3]

Further Update:
I have spoken with Plusnet today and they have promised to find out how to escalate this to BTW so that BTW can ascertain that their speedtester is compatible with the combination of Firefox version 44.0.2 and Flash Player version 20.0.0.306.
Official bug reports have been raised with Mozilla (for Firefox) - bug report 1248709 and Adobe (for Flash Player) - bug report  4118299.
Hopefully between these three approaches a solution will arise.
It seems pretty obvious to me that the "anomaly" is a result of the combination of  Firefox version 44.0.2 and Flash Player version 20.0.0.306 as it started two days ago on the iMac when it had Flash Player updated, whilst at the same time did not appear on the Macbook Pro or the Windows PC which, at that time, had the previous version of Flash Player (20.0.0.286) installed. When Flash Player was updated on the Macbook Pro and the Windows PC today to 20.0.0.306 the anomaly then appeared on them as well.
It would be interesing to know if anyone else has seen the same situation or can replicate the situation.
In the meantime if I want to use the BTW speedtester I shall run it via Safari or Chrome as it gives the same results there as various other speedtesters rather than the anomalous slow download speeds that appear to be being reported under Firefox.
Just to reiterate, under Firefox all other speedtesters give the much higher figures that BTW reports when run via Safari or Chrome - so it must be the combination of  Firefox version 44.0.2 and Flash Player version 20.0.0.306 that is the problem.

[#4]

No one seems to be commenting on this thread.
The condition still persists, Mozilla (Firefox) are obviously progressing the bug at their end and Adobe (Flash Player) have the bug report active at their end.
Maybe someone from Plusnet could check that the investigations that they said would happen at their end are being persued.
No formal fault was logged with Plusnet , the situation was all reported to them via the telephone - the on-line chat not functioning at the time - and there was no suggestion by Plusnet that I should log it as a fault at that time..

[#5]

FF44.02 is playing up on several fronts    worst update they ever did , Big problems with HTLM5 / flashplayer
Even i admit to reverting to Chrome 

[#6]

Coming a bit late to this; I only just read through the thread.
I agree that there is an issue with Firefox 44.0.2 with Flash 20.0.0.306 running the BT Wholesale speed tester. On my (Win 7 x64) system Firefox gives a substantially lower speed estimate than running the test in Internet Explorer. The IE number is consistent with the estimate from Thinkbroadband, Firefox isn't.