Forcing Firefox 54, and saving web pages as single files
The new multi-process Firefox 54 is great, but you have to remove incompatible extensions to get it to work. One of mine was an add-on to save web pages as single files, but luckily there are ways to save MHTML files using other browsers
Mozilla has spent a long time redesigning its Firefox browser to use more than one process, and it came up with a good solution in Electrolysis (e10s). Unfortunately, it's taking a long time to roll out, partly because so many add-ons are still incompatible with the new system.
Although Mozilla has just announced version 54 as a multi-process version of Firefox - as ZDNet reported - e10s has been working since 2014, and shipping in release versions since Firefox 48. However, Mozilla has been turning it on for relatively small numbers of users at a time, to make sure that it doesn't cause major problems.
Electrolysis has significant benefits. Running the whole browser in a single process makes it a single point of failure, as distinct from Google Chrome, which uses lots of processes including one for each tab. The downside is that Chrome consumes so many resources it can bring your PC almost to a halt.
With e10s, Firefox uses four processes as standard. This distributes the load to provide better performance while still using far less memory than Chrome.
I'm one of the people who should have benefited, since I typically have around 100-150 tabs in Firefox and, regrettably, once went over 600. (Thanks to "lazy loading", Firefox only loads tabs when you click them, so most were not loaded.) But I couldn't, because several of my add-ons were not e10s compatible. In other words, the suppliers had not converted them to the new WebExtensions format required by e10s. My blockers included Reverse image search by Google, Extended Statusbar and - important to me -- MAFF.
Eventually, of course, I had to upgrade, but this did not go smoothly. Even after I removed all the incompatible add-ons, Firefox still reported that the multi-process feature was disabled. Nothing on Mozilla's support pages worked, so in the end, I followed the recommendations from a page at Ghacks and used browser.tabs.remote.force-enable. (Don't try this if you still have incompatible add-ons loaded.) Incidentally, you can also use dom.ipc.processCount to select more processes.
Saving web pages as single files
Losing MAFF was a major blow. This whole century, I've been saving web pages as single files in the standard MHTML (aka MIME HTML or MHT) format. This is much cleaner than saving them as HTML with a subsidiary folder that may hold dozens of tiny files, all cluttering the place up while consuming storage space. MHTML also makes it very easy to drop web pages into online drives such as OneDrive, Google Drive and Dropbox.
Now I can't save MHTML files from Firefox, I have to copy the addresses into another browser and save them from that.
There are a number of options....
Microsoft's Internet Explorer 11 provides good support for MHTML, and it's one of the Save options. In fact, all recent versions of IE support MHTML, starting with IE5.
Google Chrome gives you to option to save in MHTML, but only if you change the setting in chrome://flags. The drawback is that this removes the option to save in HTML instead.
Opera, very sensibly, made MHTML the default format for saving web pages, but that feature was lost when it switched from its own Presto engine to Google's Chromium source code. However, Opera 16 reintroduced MHTML. To enable it, just set opera://flags#save-page-as-mhtml
The same option works in Vivaldi, which is another Chromium-based browser. In this case, set vivaldi://flags/#save-page-as-mhtml Happily, this provides both HTML and MHTML saves.
If you've been using the Mozilla Archive Format (MAFF) instead of MHTML, then you may have a problem when MAFF is no longer supported. Mozilla says that "future versions of Firefox will still support the jar: protocol for reading the contents of MAFF files without installing an add-on" but this isn't what I need. Fortunately, MAFF files are just zip files. Rename your MAFF files to .zip and you should be able to open them.
Without MAFF, we need a WebExtension that supports MHTML, so I hope someone will write one. Otherwise, I will probably shift to doing more research in Vivaldi - my other standard browser - and less in Firefox.