Update test skips and comments related to OffscreenCanvas tests in Firefox (#25496)

Update test skips and comments related to OffscreenCanvas tests in
Firefox.

Author: juj

.circleci/config.yml

@@ -1113,16 +1113,10 @@ jobs:
           # initial position of the mouse pointer relative to the canvas.
           # browser.test_html5_webgl_create_context is skipped because
           # anti-aliasing is not well supported.
-          # browser.test_webgl_offscreen_canvas_in_pthread and
-          # browser.test_webgl_offscreen_canvas_in_mainthread_after_pthread
-          # are crashing Firefox (bugzil.la/1281796). The former case is
-          # further blocked by issue #6897.
           test_targets: "
             browser
             skip:browser.test_sdl2_mouse
             skip:browser.test_html5_webgl_create_context
-            skip:browser.test_webgl_offscreen_canvas_in_pthread
-            skip:browser.test_webgl_offscreen_canvas_in_mainthread_after_pthread
             skip:browser.test_glut_glutget
             "
   test-browser-firefox-wasm64:

test/test_browser.py

@@ -4241,6 +4241,9 @@ def test_utf16_textdecoder(self):
   # Tests that it is possible to initialize and render WebGL content in a
   # pthread by using OffscreenCanvas.
   @no_chrome('https://crbug.com/961765')
+  # The non-chained version suffers from browser priority inversion deadlock problem: offscreenCanvas.getContext("webgl2") does not make progress in a pthread until main thread yields to event loop.
+  # The chained version of this test suffers from bug https://bugzilla.mozilla.org/show_bug.cgi?id=1992576
+  @no_firefox('https://bugzilla.mozilla.org/show_bug.cgi?id=1972240 (priority inversion deadlock) + https://bugzilla.mozilla.org/show_bug.cgi?id=1992576 (chained OffscreenCanvas transfer)')
   @parameterized({
     '': ([],),
     # -DTEST_CHAINED_WEBGL_CONTEXT_PASSING:
@@ -4430,6 +4433,12 @@ def test_webgl_offscreen_canvas_in_proxied_pthread(self, asyncify):
   @requires_offscreen_canvas
   def test_webgl_resize_offscreencanvas_from_main_thread(self, args1, args2, args3):
     cmd = args1 + args2 + args3 + ['-pthread', '-lGL', '-sGL_DEBUG']
+
+    if is_firefox() and '-sOFFSCREENCANVAS_SUPPORT' in cmd and '-sPROXY_TO_PTHREAD' in cmd:
+      # Firefox is unable to transfer the same OffscreenCanvas multiple times across Workers
+      # (in a chained fashion, e.g. main thread -> proxy-to-pthread main thread -> back to main thread -> user pthread)
+      self.skipTest('https://bugzilla.mozilla.org/show_bug.cgi?id=1992576')
+
     print(str(cmd))
     self.btest_exit('test_webgl_resize_offscreencanvas_from_main_thread.c', cflags=cmd)