I had no problem with the link.

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP in front of the numbers/letters, click it to submit them.

The crash report is several pages of data. We need the report numbers to see the whole report.

Using your mouse, mark the most resent 7 - 10 crash reports, and copy them. Now go to the reply box below and paste them in.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Thanks Fred and indeed Safe Mode did prevent the problem.

On reflection "crashes" was an inaccurate description by me - what happens is that the browser is redirected to this very strange page:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjHk63duuTMAhUHKx4KHSxYCg0QFggcMAA&url=http%3A%2F%2Fforum.xda-developers.com%2Fgalaxy-s4%2Fgeneral%2From-cm13-0-galaxy-s4-ve-gt-i9515-t3245133&usg=AFQjCNG2NSopzFxmOrC1h9U2_M2MDZOOOQ

I had suspected it was a Javascript problem but clearly it is related to one or more Add-ons.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Not malware but an add-on extension named Simple. It is so old I do not even recall its purpose, but now toast and all is well. Thanks for the hint to try again with add-ons disabled.