There are two extensions that "do" MHT files.

https://addons.mozilla.org/en-US/firefox/addon/mozilla-archive-format/ and https://addons.mozilla.org/en-US/firefox/addon/unmht/ UnMHT 8.3.0 does state that there was a "fix" made for Firefox 51 as far as an API change made in Firefox 51. So I assume that MAF is affected by that API change made for Firefox 51. But at the bottom of "About this Add-on" it does state that UnMHT is not compatible with e10s..

I installed UnMHT in Firefox 51.0.1 [to test it for you] - it works to save an MHT file and will read already created MHT files that were saved in the past, but Firefox automatically disabled "Multiprocess Windows = 0/1 (Disabled by add-ons) upon restart after installation; as expected, given the "not compatible with e10s" message at AMO.

Bottom line is that both MHT extensions need updating for "compatibility", or a complete rewrite to WebExtension configuration, or they'll be "gone / unsuable" by the end of this year.

My guess is that UnMHT stands a better "chance of surviving" than MAF. MAF Feedback page: http://maf.mozdev.org/feedback.html - the email link seems ro be disabled and the "Report bug or feature request" lionk goes nowhere, leaving me to assume tat developer has lost interest in continuing with MAF.

Where with UnMHT there is recent activity in their support forum. http://www.unmht.org/forum/en/

You can disable multi-process tabs in Firefox by setting the related prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

• http://www.ghacks.net/2016/07/22/multi-process-firefox/

Thank you both for very quick responses. I am now busy on other stuff for a couple of days, will probably try out one of them after that (since i do want to upgrade from 50.1).

Meanwhile I have looked at MAF since that is the add-on I have been using. I *may* have managed to send an email to their developer, but in any case their page at http://maf.mozdev.org/installation.html is fairly encouraging, it does appear that there is work under way for multi-process and their beta version already offers the partial support claimed in the message I saw.

cor-el, thank you for the ghacks.net URL which does seem to be useful information. (Of course I did not notice it when it first came out because I just use firefox as a tool, I don't normally spend time on understanding its development path. Seems as though I might have to.)

http://maf.mozdev.org/installation.html

The "Beta version" Install button on that page would download the 4.0.1rc beta version. The rc is for Release Candidate. Same as what is listed here - https://addons.mozilla.org/en-US/firefox/addon/mozilla-archive-format/versions/beta - which is dated June 11, 2016, which is the same day as the 4.0.1 release version date. The Development Channel has had no activity after that date.

Hopefully the developer will respond to your email and have good news for you.

Mozilla: shame on you for releasing a half baked v51.0.1. If you had mentioned the inability to read mht files in the popup after install I would not have wasted an hour trying to fix the problem. Better, since you obviously knew about it, had you detected my OS, extensions, etc. and disabled browser tabs as needed. Even more professional if you waited to release a working version.

Sorry that you are unhappy, but switching to the UnMHT extension might solve your current issue.

MHT / MHTML - https://en.wikipedia.org/wiki/MHTML - was never supported in Firefox. MHTML is a proposed open standard, was never made an official standard; and as such Mozilla has never supported it. It's only been available in Firefox via an add-on / extension. As such, Mozilla has no responsibility to make sure API changes don't break things. An as far as warning users upon installing an update, with 10's of thousands (or over 100,000) extensions created over the years, there is no way for the Firefox developers to keep track of which extensions are using specific API's to extend Firefox features.

As I pointed out, the developer of UnMHT planned ahead and made the necessary changes to his extension back in November 2016 - far in advance of Firefox 51 being released.

MAF still needs to be fixed.

Maybe the developer of MAF doesn't get the newsletter that Mozilla sends to registered / enrolled add-on developers months in advance of changes like this being Released, to give the developer a generous amount of time to "fix" their add-on and not cause their users problems like yours.

This is a critical bug in version 4.0.1 of the Mozilla Archive Format add-on, that I'm currently maintaining. Thanks to a rapid response from Add-on Reviewers, an automatic update to version 4.0.2 is already available to solve the issue.

The root cause was in the way the add-on declared its incompatibility with multi-process Firefox.

As of today, multi-process is only available to general Firefox users if none of the add-ons they installed declare themselves as incompatible. The method that add-on authors should use to declare incompatibility has changed in Firefox 51, hence the bug.

This change was previously undocumented, except as an aside in an internal wiki page relating to the controlled rollout of the multi-process architecture. After confirming that the new method resolved the issue, I have updated the developer-facing documentation here:

https://developer.mozilla.org/en-US/Firefox/Releases/51#Other https://developer.mozilla.org/en-US/Add-ons/Install_Manifests#multiprocessCompatible

While add-on developers have been receiving newsletters about the conversion to multi-process architecture since August 2015 at least, I didn't receive a notice of this particular change planned for January 2017. Unfortunately, the nature of this issue meant it could only be detected after the Firefox update.

For clarity, the message "multi-process is only partially supported by this version of the Mozilla Archive Format add-on" is only intended for users on pre-release channels. At the current stage of rollout, users on the Release channel with one or more incompatible add-ons should have multi-process disabled automatically, without even noticing.

While I'm here, I'll mention that full multi-process compatibility is definitely part of the future plans of the Mozilla Archive Format add-on. There is a clear technical path forward, although it takes a very long time to implement.

Longer term, I would like for users of the UnMHT and Mozilla Archive Format add-ons to be able to continue to have an integrated user experience for viewing and saving MHTML and MAFF archives.

However, I'm not sure about the technical feasibility of doing that using a WebExtension that is not marked as "experimental". If plans for supporting WebExtensions exclusively in the long term continue, the likely consequences may range from a slightly degraded user experience to a complete loss of support. There might be a way of keeping the current experience on pre-release channels, but most users won't benefit from them. On the upside, if feasibility is confirmed, it might be easier to convert the code base to support MAFF in other major browsers other than Firefox.

If none of the major browsers will allow the MAFF file format to be integrated seamlessly, I will ensure that a clear path forward towards other formats exists through the "Saved Pages Conversion Wizard" currently provided by the Mozilla Archive Format add-on.

Thank you I have today allowed the update to Firefox 51.0.1 and it is all working OK. (MAF has updated itself to 4.0.2)

Same for me on Vista Laptop MAFF 4.0.2 and FF51.0.1 now opens old mht files OK with

   browser.tabs.remote.autostart = false
   browser.tabs.remote.autostart.2 = true

Panic over.