Unfortunately I forgot to test `gulp mozcentraldiff` with PR 11256, which is really bad since it will cause the en-US `chrome.properties` l10n file to be deleted at mozilla-central; sorry about breaking this!
In order to address this we'll have to re-add (only) the en-US `chrome.properties` l10n file, which is simple enough. Furthermore, since we're not doing any sort of build-specific parsing of the l10n files, we can just copy the en-US files as-is rather than having to run `gulp locale` during `gulp mozcentral`.
With the removal of the (standalone) Firefox building code in PR 9566 (a year and a half ago), these files are now completely unused in the GitHub repository[1].
Hence it doesn't really seem necessary to keep fetching them with `gulp importl10n`, and the existing files in the `l10n` folder can also be removed (thanks to version control, they're easy enough to restore should the need ever arise).
The patch also allows an additional simplification, for the `gulp locale` and `gulp mozcentral` commands, since it's now possible to stop writing `l10n` files to the `extensions/firefox/` folder and instead just copy them similar to other build targets.
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[1] They're obviously still used in `mozilla-central`, for fallback messages displayed through `PdfStreamConverter.jsm`, but that doesn't make it necessary to keep them *here* as far as I'm concerned.
