[Firefox addon] Change the minimum supported version to (the current) Firefox Nightly release

At this point in time, trying to keep the development addon compatible with prior Firefox versions is already quite difficult and will become even harder very soon.

Please keep in mind that since Firefox 57, only WebExtensions are allowed/possible to install. The only exceptions are Firefox Nightly, with the `xpinstall.signatures.required` preference[1] set to `false`, and the (as of this writing) current ESR release.[2]

With the current compatibility situation, we thus need to effectively support both Nightly *and* ESR in the addon, while trying to keep up with current/upcoming changes in `mozilla-central`. With old addons no longer being officially supported, the amount of old code being removed/refactored is now increasing quite quickly.

*Please note:* The changes proposed here was to a large extent prompted by bugs such as:
 - https://bugzilla.mozilla.org/show_bug.cgi?id=1431533 (already landed)
 - https://bugzilla.mozilla.org/show_bug.cgi?id=767640 (already landed)
 - https://bugzilla.mozilla.org/show_bug.cgi?id=1432992 (still open)

Upstreaming all of those changes for the `MOZCENTRAL` version of PDF.js, while also keeping the Firefox addon running in older versions of the browser, would quickly become non-trivial.
Since we're using the ruleset from `eslint-plugin-mozilla` for the addon files, which is enforced in `mozilla-central`, we must ensure that the neccessary changes are upstreamed to the PDF.js repo such that the `mozilla-central` version of PDF.js can still be updated without failures.

Trying to feature detect, for the `FIREFOX` build target, some of the changes in the referenced bugs would probably become both quite messy and (not to mention) difficult. E.g. with the upcoming and automatically defined `Cc, Ci, Cu, Cr` variables, any sort of feature detection might be tricky since those need to be defined in the global scope of the files in question.

Finally, given the amount of effort that we'd now need to spend to even attempt to keep the Firefox addon compatible, I just don't think it's worth the effort any more. Especially since the number of people that have, thus far, been doing this work is *very* low and those resources would be better spend elsewhere.
Unfortunately, this probably means that the development addon will no longer be compatible with release versions of Seamonkey. However the README has already mentioned, for quite some time, that support isn't guaranteed.

*In closing:* For all of the reasons mentioned above, I thus propose that we reduce the maintenance burden of the Firefox addon by only supporting the current Firefox Nightly.

---

[1] While the preference exists, and can be toggled, its value is ignored in non-Nightly versions of Firefox.

[2] There's unbranded builds of e.g. the release/beta versions of Firefox, where old and non-WebExtensions addons can be installed. However those builds can probably be assumed to be officially unsupported, and thus not recommended for users.

README.md

@@ -33,9 +33,9 @@ PDF.js is built into version 19+ of Firefox, however, one extension is still ava
 
 + [Development Version](http://mozilla.github.io/pdf.js/extensions/firefox/pdf.js.xpi) - This extension is mainly intended for developers/testers, and it is updated every time new code is merged into the PDF.js codebase. It should be quite stable but might break from time to time.
 
-  + Please note that the extension is *not* guaranteed to be compatible with Firefox versions that are *older* than the current ESR version, see the [Release Calendar](https://wiki.mozilla.org/RapidRelease/Calendar#Past_branch_dates).
+  + Please note that the extension is *not* guaranteed to be compatible with Firefox versions that are *older* than the current Nightly version, see the [Release Calendar](https://wiki.mozilla.org/RapidRelease/Calendar#Past_branch_dates).
 
-  + The extension should also work in Seamonkey, provided that it is based on a Firefox version as above (see [Which version of Firefox does SeaMonkey 2.x correspond with?](https://wiki.mozilla.org/SeaMonkey/FAQ#General)), but we do *not* guarantee compatibility.
+  + The extension *may* also work in Seamonkey, provided that it is based on a Firefox version as above (see [Which version of Firefox does SeaMonkey 2.x correspond with?](https://wiki.mozilla.org/SeaMonkey/FAQ#General)), but we do *not* guarantee compatibility.
 
 #### Chrome
 

extensions/firefox/install.rdf

@@ -13,8 +13,8 @@
     <em:targetApplication>
       <Description>
        <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
-       <em:minVersion>45.0</em:minVersion>
+       <em:minVersion>59.*</em:minVersion>
-       <em:maxVersion>54.0a1</em:maxVersion>
+       <em:maxVersion>61.0a1</em:maxVersion>
      </Description>
     </em:targetApplication>
 
@@ -31,8 +31,8 @@
     <em:targetApplication>
       <Description>
        <em:id>{aa3c5121-dab2-40e2-81ca-7ea25febc110}</em:id>
-       <em:minVersion>45.0</em:minVersion>
+       <em:minVersion>59.*</em:minVersion>
-       <em:maxVersion>54.0a1</em:maxVersion>
+       <em:maxVersion>61.0a1</em:maxVersion>
      </Description>
     </em:targetApplication>
 

extensions/firefox/update.rdf

@@ -14,8 +14,8 @@
             <em:targetApplication>
               <RDF:Description>
                 <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
-                <em:minVersion>45.0</em:minVersion>
+                <em:minVersion>59.*</em:minVersion>
-                <em:maxVersion>54.0a1</em:maxVersion>
+                <em:maxVersion>61.0a1</em:maxVersion>
                 <!-- Use the raw link for updates so we we can use SSL. -->
                 <em:updateLink>https://raw.githubusercontent.com/mozilla/pdf.js/gh-pages/extensions/firefox/pdf.js.xpi</em:updateLink>
               </RDF:Description>
@@ -25,8 +25,8 @@
             <em:targetApplication>
               <RDF:Description>
                 <em:id>{aa3c5121-dab2-40e2-81ca-7ea25febc110}</em:id>
-                <em:minVersion>45.0</em:minVersion>
+                <em:minVersion>59.*</em:minVersion>
-                <em:maxVersion>54.0a1</em:maxVersion>
+                <em:maxVersion>61.0a1</em:maxVersion>
                 <!-- Use the raw link for updates so we we can use SSL. -->
                 <em:updateLink>https://raw.githubusercontent.com/mozilla/pdf.js/gh-pages/extensions/firefox/pdf.js.xpi</em:updateLink>
               </RDF:Description>