https://dxr.mozilla.org/mozilla-central/rev/a6a1f5c1d971dbee67ba6eec7ead7902351ddca2/browser/components/customizableui/CustomizableUI.jsm#370-373 savedPanelPlacements = savedPanelPlacements.filter(id => defaultPlacements.includes(id)); if (savedPanelPlacements.length) { gSavedState.placements[this.AREA_FIXED_OVERFLOW_PANEL] = savedPanelPlacements; } There are two issues here. 1. that filter() is the wrong way around. 2. `this.AREA_FIXED_OVERFLOW_PANEL` doesn't exist, because it lives on the public CustomizableUI object, and `this` is the internal object. I will see if I can write an automated test for this.

Comment on attachment 8898786 [details] Bug 1391616 - make migration of existing hamburger panel contents actually work, https://reviewboard.mozilla.org/r/170176/#review175314 ::: browser/components/customizableui/test/browser_1042100_default_placements_update.js:111 (Diff revision 1) > + let springs = navbarPlacements.filter(id => id.includes("spring")); > + is(springs.length, 2, "Should have 2 toolbarsprings in placements now"); The ids of the springs are unpredictable, so I'm checking this separately. This patch will conflict with the download button work in bug 1371765, I'll deal with the fallout. I'm verifying the button placements one-by-one so that things won't break if we end up with system add-ons or further migrations manipulating the contents of the navbar further.

Comment on attachment 8898786 [details] Bug 1391616 - make migration of existing hamburger panel contents actually work, https://reviewboard.mozilla.org/r/170176/#review175474 ::: browser/components/customizableui/test/browser_1042100_default_placements_update.js:113 (Diff revision 1) > + navbarPlacements = navbarPlacements.filter(id => !id.includes("spring")); > + is(navbarPlacements[0], "back-button", "Back button is in the right place."); Can you also assert that the length is 7?

Pushed by gijskruitbosch@gmail.com: https://hg.mozilla.org/integration/autoland/rev/6810c28dcb30 make migration of existing hamburger panel contents actually work, r=jaws

Since Gijs is out on PTO -- Jared, do you have suggested STR for this issue for bug verification purposes? Thanks.

(In reply to Grover Wimberly IV [:Grover-QA] from comment #7) > Since Gijs is out on PTO -- Jared, do you have suggested STR for this issue > for bug verification purposes? Thanks. 1. open a new profile on 56 or earlier 2. open customize mode 3. add some items to the hamburger panel that are in the navbar or the palette (toolbox / extra/unused items area) by default 4. exit customize mode 5. close browser 6. open 57/58 with that profile Expected: any items added to the hamburger that weren't in the hamburger panel originally should appear in the pinned part of the overflow panel Pre-patch: they disappeared to the palette (so only visible when opening customize mode). Does that help clarify?

Verified on Windows 10 64bit, Mac OS X 10.11, Ubuntu 16.04 64bit using Nightly 58.0a1 (64-bit) as of this date.

I have verified that the issue mentioned in comment 8 is no longer reproducible using Firefox 57.0b9 (BuildId:20171016185129) on Windows 10 64bit, macOS 10.12.1 and Ubuntu 16.04 64bit.