postmarketos-demos: Depend on firefox-esr instead of firefox

The firefox package is currently broken in armhf Alpine,
and it looks like fixing will take a while (depends on packaging
rust for armhf on Alpine):

https://bugs.alpinelinux.org/issues/7409
https://github.com/alpinelinux/aports/pull/1578#discussion_r119988654

postmarketos-demos/APKBUILD

@@ -1,11 +1,11 @@
 pkgname=postmarketos-demos
 pkgver=3
-pkgrel=1
+pkgrel=2
 pkgdesc="Simple touch menu for a few demo programs"
 url="https://github.com/postmarketOS"
 arch="all"
 license="GPL3+"
-depends="weston-clients firefox"
+depends="weston-clients firefox-esr"
 makedepends="gtk+3.0-dev"
 subpackages=""
 source="main.c Makefile"