diff --git a/apps/m9zCtrlTty/firefox_install.go b/apps/m9zCtrlTty/firefox_install.go
index 01c16dc..22de4f6 100644
--- a/apps/m9zCtrlTty/firefox_install.go
+++ b/apps/m9zCtrlTty/firefox_install.go
@@ -55,11 +55,29 @@ func isFirefoxUsable() bool {
 }
 
 func isFirefoxInstalled() bool {
-	cmd := exec.Command("which", "firefox")
-	out, err := cmd.CombinedOutput()
-	ok := err == nil
-	kioskDebugf("firefox install path check ok=%v output=%q err=%v", ok, strings.TrimSpace(string(out)), err)
-	return ok
+	if path, err := exec.LookPath("firefox"); err == nil {
+		kioskDebugf("firefox install path check ok=true path=%s source=PATH", path)
+		return true
+	}
+
+	for _, path := range []string{
+		"/usr/bin/firefox",
+		"/usr/local/bin/firefox",
+		"/snap/bin/firefox",
+	} {
+		if isExecutableFile(path) {
+			kioskDebugf("firefox install path check ok=true path=%s source=fallback", path)
+			return true
+		}
+	}
+
+	kioskDebugf("firefox install path check ok=false")
+	return false
+}
+
+func isExecutableFile(path string) bool {
+	info, err := os.Stat(path)
+	return err == nil && !info.IsDir() && info.Mode()&0111 != 0
 }
 
 func installFirefox() error {
diff --git a/apps/m9zCtrlTty/install.go b/apps/m9zCtrlTty/install.go
index c995d76..3641bf6 100644
--- a/apps/m9zCtrlTty/install.go
+++ b/apps/m9zCtrlTty/install.go
@@ -7,20 +7,33 @@ import (
 )
 
 func install() error {
-	kioskInfof("checking firefox usability")
+	kioskInfof("checking firefox installation")
 	var firefoxInstallErr error
-	if isFirefoxUsable() {
-		kioskInfof("firefox is usable")
+
+	firefoxInstalled := isFirefoxInstalled()
+	firefoxUsable := false
+	if firefoxInstalled {
+		firefoxUsable = isFirefoxUsable()
+	}
+	markerExists := hasFirefoxInstallMarker()
+	kioskInfof("firefox startup state installed=%v usable=%v markerExists=%v", firefoxInstalled, firefoxUsable, markerExists)
+
+	switch {
+	case firefoxInstalled && firefoxUsable:
+		kioskInfof("firefox installed and usable")
 		fmt.Println("Firefox is installed and usable")
 		if err := writeFirefoxInstallMarker("usable"); err != nil {
 			kioskWarnf("write firefox install marker failed err=%v", err)
 		}
-	} else if hasFirefoxInstallMarker() {
-		kioskWarnf("firefox unusable but install marker exists; skip automatic repair")
-		fmt.Println("Firefox unusable; skip automatic repair on startup")
-	} else {
-		kioskWarnf("firefox unusable and no install marker; starting first-time install")
-		fmt.Println("Firefox missing or unusable, installing...")
+	case firefoxInstalled:
+		kioskWarnf("firefox is installed but version check failed; skip automatic reinstall")
+		fmt.Println("Firefox installed but unusable; skip automatic reinstall on startup")
+	case markerExists:
+		kioskWarnf("firefox executable missing but install marker exists; skip automatic reinstall")
+		fmt.Println("Firefox missing; skip automatic reinstall on startup")
+	case shouldAutoInstallFirefox(firefoxInstalled, markerExists):
+		kioskWarnf("firefox executable missing and no install marker; starting first-time install")
+		fmt.Println("Firefox missing, installing...")
 		if err := installFirefox(); err != nil {
 			kioskErrorf("firefox first-time install failed err=%v", err)
 			fmt.Printf("install Firefox failed: %v\n", err)
@@ -59,3 +72,7 @@ func install() error {
 	}
 	return nil
 }
+
+func shouldAutoInstallFirefox(installed bool, markerExists bool) bool {
+	return !installed && !markerExists
+}
diff --git a/apps/m9zCtrlTty/install_test.go b/apps/m9zCtrlTty/install_test.go
new file mode 100644
index 0000000..517e74c
--- /dev/null
+++ b/apps/m9zCtrlTty/install_test.go
@@ -0,0 +1,39 @@
+package main
+
+import "testing"
+
+func TestShouldAutoInstallFirefox(t *testing.T) {
+	tests := []struct {
+		name         string
+		installed    bool
+		markerExists bool
+		want         bool
+	}{
+		{
+			name:         "missing without marker can install",
+			installed:    false,
+			markerExists: false,
+			want:         true,
+		},
+		{
+			name:         "installed must not reinstall even when unusable",
+			installed:    true,
+			markerExists: false,
+			want:         false,
+		},
+		{
+			name:         "missing with marker must not reinstall",
+			installed:    false,
+			markerExists: true,
+			want:         false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := shouldAutoInstallFirefox(tt.installed, tt.markerExists); got != tt.want {
+				t.Fatalf("shouldAutoInstallFirefox(%v, %v) = %v, want %v", tt.installed, tt.markerExists, got, tt.want)
+			}
+		})
+	}
+}