It's me answering my question but will post in case it helps someone else. I think that the version of Firefox that I have (39) is not compatible with the UnMHT add-on that is presently available. I believe that there is a later version of UnMHT that is going to be the basis of a revised add-on at some stage. Not a techie so hope this makes sense and hope it helps. :-)

https://addons.mozilla.org/en-US/firefox/addon/unmht/

About this Add-on NOTE: Ver.7.3.0.5 isn't compatible with Firefox 39 or later. Ver.8.0.0 is waiting for getting review.

Version 8.0.0.0 is available here: https://addons.mozilla.org/en-US/firefox/addon/unmht/versions/

Hi there all.

I just wanted to let Upplander know that he is not alone with the said problem. I have UnMHT 7.3.0.5.1-signed. I just attempted to save as MHT only to find, like Upplander, that the option was no longer given under the file menu, but neither was there any sign of the extension in the browser (as an icon at the top right of the screen or in the customize tab) except under 'Add-ons'. I have been using UnMHT for years now and so I have a lot of MHT files; thankfully, I have found that FF 39.0 will at least open my existing MHT files so it is evidentally not entirely incompatible with the extension, but I do hope that a fully compatible update is available soon.

Why has Mozilla rendered UnMHT incompatible at all? Is there any reason why they cannot make it compatible again? Is this a choice or an unintended side-effect?

All the best,

Ian.

Thanks Ian. I too use MHT a great deal so was miffed to lose the facility. I followed the link from the-edmeister, downloaded same and all is working - I can save MHTs and all the existing ones work fine.

I assume that "awaiting review" simply means that there is a process of approval for new add-ons which protects users of Firefox. At least you can download a "yet to be approved" add-on and if you are willing to take that risk then this does offer a sensible way forward this side of approval.

Regards

Upplander

Upplander, hi.

Thanks for information. I think I will Mozilla a little while to check out the stability of the version 8 add-on, but if it takes a little too long I will go ahead and install it.

Thanks also to the-edmeister for the reference.

Ian.