diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index 957d7b1283a9..84c07e34e2ef 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -3879,6 +3879,26 @@ toolbar#nav-bar {
         self.log.warning("Force-terminating active process(es).")
 
         browser_pid = browser_pid or proc.pid
+
+        # Send a signal to start the profiler - if we're running on Linux or MacOS
+        if mozinfo.isLinux or mozinfo.isMac:
+            self.log.info("Sending SIGUSR1 to pid %d start the profiler." % browser_pid)
+            os.kill(browser_pid, signal.SIGUSR1)
+            self.log.info("Waiting 10s to capture a profile.")
+            time.sleep(10)
+            self.log.info("Sending SIGUSR2 to pid %d stop the profiler." % browser_pid)
+            os.kill(browser_pid, signal.SIGUSR2)
+            # We trigger `killPid` further down in this function, which will
+            # stop the profiler writing to disk. See Bug 1906151 for more
+            # details, andBug 1905929 for an intermediate solution that would
+            # allow this test to watch for the profile file being completed.
+            self.log.info("Wait 10s for Firefox to write the profile to disk.")
+            time.sleep(10)
+        else:
+            self.log.info(
+                "Not sending a signal to start the profiler - not on MacOS or Linux. See Bug 1823370."
+            )
+
         child_pids = self.extract_child_pids(processLog, browser_pid)
         self.log.info("Found child pids: %s" % child_pids)
 
diff --git a/tools/profiler/core/platform.cpp b/tools/profiler/core/platform.cpp
index 8087184b2a3d..bbb8713e4f07 100644
--- a/tools/profiler/core/platform.cpp
+++ b/tools/profiler/core/platform.cpp
@@ -941,7 +941,7 @@ class CorePS {
   PS_GET_AND_SET(const nsACString&, ProcessName)
   PS_GET_AND_SET(const nsACString&, ETLDplus1)
 #if !defined(XP_WIN)
-  PS_GET_AND_SET(const Maybe<nsCOMPtr<nsIFile>>&, DownloadDirectory)
+  PS_GET_AND_SET(const Maybe<nsCOMPtr<nsIFile>>&, AsyncSignalDumpDirectory)
 #endif
 
   static void SetBandwidthCounter(ProfilerBandwidthCounter* aBandwidthCounter) {
@@ -1007,7 +1007,7 @@ class CorePS {
 
   // Cached download directory for when we need to dump profiles to disk.
 #if !defined(XP_WIN)
-  Maybe<nsCOMPtr<nsIFile>> mDownloadDirectory;
+  Maybe<nsCOMPtr<nsIFile>> mAsyncSignalDumpDirectory;
 #endif
 };
 
@@ -5653,10 +5653,10 @@ Maybe<nsAutoCString> profiler_find_dump_path() {
     // Acquire the lock so that we can get things from CorePS
     PSAutoLock lock;
     Maybe<nsCOMPtr<nsIFile>> downloadDir = Nothing();
-    downloadDir = CorePS::DownloadDirectory(lock);
+    downloadDir = CorePS::AsyncSignalDumpDirectory(lock);
 
     // This needs to be done within the context of the lock, as otherwise
-    // another thread might modify CorePS::mDownloadDirectory while we're
+    // another thread might modify CorePS::mAsyncSignalDumpDirectory while we're
     // cloning the pointer.
     if (downloadDir) {
       nsCOMPtr<nsIFile> d;
@@ -6848,12 +6848,12 @@ bool profiler_is_paused() {
 }
 
 // See `ProfilerControl.h` for more details.
-void profiler_lookup_download_directory() {
+void profiler_lookup_async_signal_dump_directory() {
 // This implementation is causing issues on Windows (see Bug 1890154) but as it
 // only exists to support the posix signal handling (on non-windows platforms)
 // we can remove it for now.
 #if !defined(XP_WIN)
-  LOG("profiler_lookup_download_directory");
+  LOG("profiler_lookup_async_signal_dump_directory");
 
   MOZ_ASSERT(
       NS_IsMainThread(),
@@ -6864,16 +6864,45 @@ void profiler_lookup_download_directory() {
 
   // take the lock so that we can write to CorePS
   PSAutoLock lock;
+  nsresult rv;
 
-  nsCOMPtr<nsIFile> tDownloadDir;
-  nsresult rv = NS_GetSpecialDirectory(NS_OS_DEFAULT_DOWNLOAD_DIR,
-                                       getter_AddRefs(tDownloadDir));
-  if (NS_FAILED(rv)) {
-    LOG("Failed to find download directory. Profiler signal handling will not "
-        "be able to save to disk. Error: %s",
-        GetStaticErrorName(rv));
+  // Check to see if we have a `MOZ_UPLOAD_DIR` first - i.e., check to see if
+  // we're running in CI.
+  LOG("Checking if MOZ_UPLOAD_DIR exists");
+  const char* mozUploadDir = getenv("MOZ_UPLOAD_DIR");
+  if (mozUploadDir && mozUploadDir[0] != '\0') {
+    LOG("Found MOZ_UPLOAD_DIR at: %s", mozUploadDir);
+    // We want to do the right thing, and turn this into an nsIFile. Go through
+    // the motions here:
+    nsCOMPtr<nsIFile> mozUploadDirFile =
+        do_CreateInstance("@mozilla.org/file/local;1", &rv);
+
+    if (NS_FAILED(rv)) {
+      LOG("Failed to create nsIFile for MOZ_UPLOAD_DIR: %s, Error: %s",
+          mozUploadDir, GetStaticErrorName(rv));
+    } else {
+      rv = mozUploadDirFile->InitWithNativePath(
+          nsDependentCString(mozUploadDir));
+      if (NS_FAILED(rv)) {
+        LOG("Failed to assign a filepath while creating MOZ_UPLOAD_DIR file "
+            "%s, Error %s ",
+            mozUploadDir, GetStaticErrorName(rv));
+      } else {
+        CorePS::SetAsyncSignalDumpDirectory(lock, Some(mozUploadDirFile));
+      }
+    }
   } else {
-    CorePS::SetDownloadDirectory(lock, Some(tDownloadDir));
+    LOG("Defaulting to the user's Download directory for profile dumps")
+    nsCOMPtr<nsIFile> tDownloadDir;
+    rv = NS_GetSpecialDirectory(NS_OS_DEFAULT_DOWNLOAD_DIR,
+                                getter_AddRefs(tDownloadDir));
+    if (NS_FAILED(rv)) {
+      LOG("Failed to find download directory. Profiler signal handling will "
+          "not be able to save to disk. Error: %s",
+          GetStaticErrorName(rv));
+    } else {
+      CorePS::SetAsyncSignalDumpDirectory(lock, Some(tDownloadDir));
+    }
   }
 #endif
 }
diff --git a/tools/profiler/public/ProfilerControl.h b/tools/profiler/public/ProfilerControl.h
index 53fec681ceda..62b9ed3e0d21 100644
--- a/tools/profiler/public/ProfilerControl.h
+++ b/tools/profiler/public/ProfilerControl.h
@@ -40,7 +40,7 @@ static inline void profiler_init(void* stackTop) {}
 static inline void profiler_shutdown(
     IsFastShutdown aIsFastShutdown = IsFastShutdown::No) {}
 
-static inline void profiler_lookup_download_directory() {}
+static inline void profiler_lookup_async_signal_dump_directory() {}
 
 #else  // !MOZ_GECKO_PROFILER
 
@@ -138,7 +138,7 @@ void profiler_ensure_started(
 // third option, by giving us a hook to look for the download directory when
 // the time is right. This might be triggered internally (e.g. when we start
 // profiling), or externally, e.g. after the directory service is initialised.
-void profiler_lookup_download_directory();
+void profiler_lookup_async_signal_dump_directory();
 
 //---------------------------------------------------------------------------
 // Control the profiler
diff --git a/xpcom/build/XPCOMInit.cpp b/xpcom/build/XPCOMInit.cpp
index 6275ca9c53d9..df51aa99f767 100644
--- a/xpcom/build/XPCOMInit.cpp
+++ b/xpcom/build/XPCOMInit.cpp
@@ -469,7 +469,7 @@ NS_InitXPCOM(nsIServiceManager** aResult, nsIFile* aBinDirectory,
   // Now that both the profiler and directory services have been started
   // we can find the download directory, where the profiler can write
   // profiles if necessary
-  profiler_lookup_download_directory();
+  profiler_lookup_async_signal_dump_directory();
 
   // Init mozilla::SharedThreadPool (which needs the service manager).
   mozilla::SharedThreadPool::InitStatics();