In https://bugzilla.mozilla.org/show_bug.cgi?id=1355216, the *third* parameter of `Services.obs.addObserver` was made optional.
However, omitting it in Firefox versions *without* that patch causes failures that completely prevents the addon from working (it won't even load).
As far as I can tell, there isn't *any* way to detect ahead of time if the third parameter can be safely omitted, hence we're forced to fallback to manually checking the version number :-(
*Note:* Since the `PdfJs.jsm` file is only used in the `MOZCENTRAL` build, we at least don't need to add any compatibility hacks there.
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