As far as I can tell, `headerURL` might already have stopped working in Firefox **70** both on Desktop and on Nightly - while I haven't tested the very last Nightly version before the branch to Firefox 71, in the local builds of Firefox 70 I still had lying around, themes with `headerURL` had already stopped working properly. Plus in any case for the current Firefox for Android there are [no official releases later than 68](https://docs.google.com/document/d/1oRPkwP3l7QzdQYj0Wn7d_3EfTZaakocA_i_pGKlG0dI/), so for "Firefox for Android" I'd just note it as 65 - 68, and for desktop I'd double-check when exactly it stopped working.
Bug 1548776 Comment 39 Edit History
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As far as I can tell, `headerURL` might already have stopped working in Firefox **70** both on Desktop and on Nightly - while I haven't tested the very last Nightly version before the branch to Firefox 71, in the local builds of Firefox 70 I still had lying around, themes with `headerURL` have already stopped working properly. Plus in any case for the current Firefox for Android there are [no official releases later than 68](https://docs.google.com/document/d/1oRPkwP3l7QzdQYj0Wn7d_3EfTZaakocA_i_pGKlG0dI/), so for "Firefox for Android" I'd just note it as 65 - 68, and for desktop I'd double-check when exactly it stopped working.