Support for <link rel="preload"> was added in Firefox 56, but then it was disabled in Firefox 57 "because of various web compatibility issues." We should verify that this feature remains disabled in ESR60 or we should ensure that it is subject to first-party isolation.
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AFAIK this is controlled via network.preload pref and it is still false in current nightly (68). So I would assume that it will be still disabled for next ESR release.
In any case, I enabled the pref in current Tor Browser and tested different kinds of 'link-rel preloads' (audio, video, script, style, fetch...), and could not find a way to make them bypass the FPI.