https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

I do not have Advanced SystemCare software installed or anything similar.

Your response implies that mm vs inches is a preference that I can set. If so, then please show me where I can set this preference.

See also:

• https://support.mozilla.org/kb/pages-appear-tiny-when-i-print-or-print-preview-firefox

I don't have any problem printing in Firefox. I can set the margins to whatever size I want and Firefox prints the page correctly with exactly the margins I've set.

Originally (up to about version 45.0) in the Page Setup... box the margins were set in inches. Now (about version 46.0 and up) the margins must be set in millimeters.

I want to know if it's possible to change Firefox back to setting the margins in inches in version 47.0.

US paper sizes use fractional inch settings.

International (A-type) paper uses metric settings.

And if you see weird settings like "0.0593..." for margins, you have corruption and all the "printer" settings should be cleared out.

Checked all printer prefs (Epson CX3810, PrimoPDF, Quicken PDF Printer, Microsoft XPS Document Writer, FAX) for paper size. All are set to "Letter".

In FF, did not see any "weird settings" for margins. All were set to ".5".

In FF, reset all prefs with "print, printer, margins, or paper" to default.

Closed FF and rebooted PC.

Opened FF, Page Setup - margins are still in millimeters!

Apparently the solution to this problem is unknown.

Hmm, my Page Setup dialog, Margins & Header/Footer tab, changed to millimeters. I hadn't noticed. Since I use the U.S. English locale, this isn't what I expect.

If I go into about:config and filter using printer*margin in the search box, those values are still in inches.

Puzzling.

When I started digging into the source code, it led me to a nsIPrintSettings object, and a source file named nsIPrintSettings.idl which had a change in Firefox 46 in this bug:

Bug #1238964 – Printing via parent doesn't handle paper sizes correctly.

Perhaps something in that patch broke the part which determines your default measurement units. But I have no idea how to fix it. It's probably best to file a new bug.

You can do that here: https://bugzilla.mozilla.org/enter_bug.cg

I clicked Firefox and then entered this as the proposed bug name:

Page Setup Margin Widths use Millimeters Instead of Inches

There were several search results listed below that, but none seemed to be the same, so either this hasn't been filed yet or it was titled so differently that I didn't get a match.

If you do file a bug, please post a link to it in this thread and/or post a link to this thread in your description of the bug.

OK, I submitted a bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1280159

In that Bug report - about:config they are shown in inches (0.393750011920929" which is a close approximation to 10mm) - which is what I was referring to in my 06-10 answer when I mentioned "0.593..." I was off a bit in my recollection of the corrupt value that has been mentioned frequently.

That is an indication of a corrupt preference that we have been seeing mention of for a few years now. IOW, not necessarily related to Firefox 47.0 specifically. There is another 'glitch' that came about with Fx46 which is related to a 'fix' for e10s compatibility (Security: Process Sandboxing), but I don't think it is related to your problem at this time. https://bugzilla.mozilla.org/show_bug.cgi?id=1156742 - which was reported by the same person who filed the Bug report that Jefferson cited, but I don;t think that is related to your problem, as I mentioned.

Not sure how you reset all the margin preferences, but see this support article. https://support.mozilla.org/en-US/kb/fix-printing-problems-firefox#w_reset-all-firefox-printer-settings

Reset all preferences that are related to that specific "printer" by carefully removing the applicable lines from the prefs.js file that is in your Profile folder. You need to close Firefox and make sure the firefox.exe process is closed in the Task Manager before editing that file, otherwise that corruption is liable to reoccur.

The issue I see is not that Firefox gets the value wrong; as shown in the attached screenshot, the converted value is correct and it is what Firefox follows.

The issue for me is that the Page Setup dialog doesn't use the units my version of Windows is set for (inches) and therefore is inconvenient for me (and other users not well versed in metric system conversions). I assume this change was an unintended byproduct of earlier changes so hopefully will get fixed without a big debate.

I removed all references to my printer from the prefs.js file as the-edmeister suggested. However, upon restarting FF, the margins are still in mm in the Page Setup dialog box and are now set to the default value 12.7mm.

When I reset the margins to 10mm, the prefs.js file shows them as 0.393750011920929. This is close to 10mm in inches but it is not correct. The correct value is 0.393700787401575. This may be nit picking but if you're going to state the value to 15 decimals you ought to do it correctly.

IMO, that 15 decimals is an example of corruption in that preference.

As an example of the limitations for prefs like that, I have never been able to set a margin to 3/8" by using 0.375" in Firefox (as I was able to do do years ago with OpenOffice) and trying it just now in Page Setup I can't get more than 1 digit to stick. For me when trying to set 0.375" reverts to 0.4" after closing the Page Setup dialog window. As far as what metric settings are possible, I don't believe Firefox is is made to have that precise control over that type of preference to go to 15 digits - that's why I think that is corruption.

Try a new Profile to see if that 15 digits 'thing' occurs in a new Profile. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Using a new profile produced the same results: margins are still in mm in the Page Setup dialog box and inches in the prefs.js file. Changing margins from the default 12.7 (0.5 in prefs.js) produced the identical error in the prefs.js file as previously reported.

Your Bug report was confirmed by a triage tester and is reproducible, plus the regression range has been identified. https://bugzilla.mozilla.org/show_bug.cgi?id=1280159#c2 Evidently the MS XPS Document Writer is what triggers it.

Related to the Bug report that I cited above. Linked to https://bugzilla.mozilla.org/show_bug.cgi?id=1238964, which was the result of https://bugzilla.mozilla.org/show_bug.cgi?id=1156742 - the 'fix' for e10s.

OK - this is getting way over my head!

Does this mean there is a fix coming?

FYI I don't have the MS XPS Document Writer installed. I did have it when I last installed Win7 (about a year ago) but I removed it since I never use it.

Did you try a new Profile as was asked about in your Bug report? https://bugzilla.mozilla.org/show_bug.cgi?id=1280159#c1

If you don't use a "printer" there should be no preferences for that "printer" appearing in either about:config or prefs.js.

As far as when a fix will be available, please wait until BobOwen responds in your Bug report. BobOwen filed the original Bug report that brought about the changes which (it seems) may have created that regression, so I expect that he would be able to answer as to "when".

Yes, I tried a new profile and nothing changed. (See my 6/15/16, 3:26 PM post.) I didn't respond on the Bug report because the next comment said "Nevermind..." before I had a chance to respond.

I note that the Bug report now says the problem is with US Legal size paper but I've had the problem with my printer set to US Letter size paper. Is this significant? Should I be making my comments here or on the Bug report or both?

I appreciate your taking the time to guide me through this. This is totally unfamiliar territory for me.

I wouldn't say anything in the bug until the person most familiar with that part of the Firefox program code has a chance to chime in.