fix(browsers): PDFs open in Firefox, not Chromium

modules/browsers.nix

@@ -130,12 +130,14 @@ in
 
   # Both browsers claim x-scheme-handler/http in their desktop entries, and
   # with two candidates xdg-open picks by whatever order it finds them in.
-  # These four types are exactly what firefox.desktop declares.
+  # The first four types are exactly what firefox.desktop declares; PDFs go to
+  # Firefox's built-in viewer as well.
   xdg.mime.defaultApplications = {
     "text/html" = "firefox.desktop";
     "application/xhtml+xml" = "firefox.desktop";
     "x-scheme-handler/http" = "firefox.desktop";
     "x-scheme-handler/https" = "firefox.desktop";
+    "application/pdf" = "firefox.desktop";
   };
 
   environment.systemPackages = [