nongnu: firefox: Use rust-1.77.

Efraim was kind enough to add rust versions up to the current 1.79 into the
Guix, however the default version was not changed yet.  That means we can drop
our firefox-rust-1.76 and just use regular rust-1.77.

* nongnu/packages/mozilla.scm (firefox-rust-1.76): Delete variable.
(rust-firefox): Define as `rust-1.77'.

Signed-off-by: Jonathan Brielmaier <jonathan.brielmaier@web.de>

nongnu/packages/mozilla.scm

@@ -72,16 +72,6 @@
   #:use-module (gnu packages xdisorg)
   #:use-module (gnu packages xorg))
 
-(define firefox-rust-1.76
-  (let ((base-rust ((@@ (gnu packages rust) rust-bootstrapped-package)
-                    rust-1.75 "1.76.0"
-                    "08f06shp6l72qrv5fwg1is7yzr6kwj8av0l9h5k243bz781zyp4y")))
-    (package
-      (inherit base-rust)
-      ;; Need llvm >= 16.0
-      (inputs (modify-inputs (package-inputs base-rust)
-                             (replace "llvm" llvm-17))))))
-
 ;;; Define the versions of rust needed to build firefox, trying to match
 ;;; upstream.  See table at [0], `Uses' column for the specific version.
 ;;; Using `rust' will likely lead to a newer version then listed in the table,
@@ -89,9 +79,8 @@
 ;;; it is a tradeoff worth making.
 ;;; 0: https://firefox-source-docs.mozilla.org/writing-rust-code/update-policy.html
 (define-public rust-firefox-esr rust)
-;; The released official binary uses 1.77, but larger changes are required,
-;; and since 1.76 is supported as well, us it.
-(define-public rust-firefox firefox-rust-1.76)
+;; The `rust' package is too old.
+(define-public rust-firefox rust-1.77)
 
 (define icu4c-73
   (package