The synced tabs sidebar ( https://searchfox.org/mozilla-central/source/browser/components/syncedtabs/sidebar.xhtml ) uses browser.dtd for its strings:

https://searchfox.org/mozilla-central/rev/c07aaf12f13037c1f5a343d31f8291549e57373f/browser/locales/en-US/chrome/browser/browser.dtd#891-924

It should instead have its own FTL file.

The tricky bit is that some strings are used in the context menu for this sidebar in browser.xul .

To make those work, we'll need to insert the FTL file dynamically when the sidebar is shown. We can do this by calling MozXULElement.insertFTLIfNeeded() on the toplevel window from within sidebar.js ( https://searchfox.org/mozilla-central/source/browser/components/syncedtabs/sidebar.js ) when the page loads.

Backed out changeset c0996caf797f (Bug 1523761) for bc failures in browser/components/customizableui/test/browser_947914_button_history.js CLOSED TREE

Failure: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&classifiedState=unclassified&fromchange=af8bce2330e926f71d6010dc02897ecb29b9191f&selectedJob=234583292

Log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=234583292&repo=autoland&lineNumber=13351

https://hg.mozilla.org/mozilla-central/rev/ff28f10d2759