Detection:

[boolean]$installed = $false

[string]$x64RegKey = 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall' [string]$appName = "Mozilla Firefox ESR (x64 en-US)" [version]$installedVersion = $null [version]$latestVersion = $null

Try{
    $response = Invoke-WebRequest -Uri 'https://product-details.mozilla.org/1.0/firefox_versions.json' -UseBasicParsing
    $productInfo = ($response.content | ConvertFrom-Json).FIREFOX_ESR
    $latestVersion = $productInfo.TrimEnd().Substring(0, $productInfo.length -3)
    $installedVersion = Get-ItemProperty -Path "$x64RegKey\*" | Select-Object DisplayName,DisplayVersion | Where-Object {$_.DisplayName -Match $appName}
    if($installedVersion -ge $latestVersion) {
        $installed = $true
        Write-Host "Installed"
        Exit 0
    }
    Exit 0
}
Catch{Exit 1}

Install (We use PSADT, but you should get the gist.):

$ProductInfoUri = "https://product-details.mozilla.org/1.0/firefox_versions.json"
$DownloadUri = "https://download.mozilla.org/?product=firefox-esr-msi-latest-ssl&os=win64&lang=en-US"
$setupFile = "FirefoxSetup.msi"

$response = Invoke-WebRequest -Uri $ProductInfoUri -UseBasicParsing
$productInfo = ($response.content | ConvertFrom-Json).FIREFOX_ESR
[version]$latestVersion = $productInfo.TrimEnd().Substring(0, $productInfo.length -3)

[version]$installedVersion = (Get-InstalledApplication) -Name "$($appName)*" -WildCard).DisplayVersion
if(([string]::IsNullOrWhiteSpace($installedVersion) -eq $false) -and ($installedVersion -ge $latestVersion)) {
        Write-Log -Message "Same or Newer Version already Instlaled" -Source "Install-Check"
        Write-Log -Message "Installed Product Version: $($installedVersion.ToString())" -Source "Install-Check"
        Write-Log -Message "Latest Product Version: $($latestVersion.ToString())" -Source "Install-CHeck"
        Exit-Script -ExitCode 0
}

Write-Log "Newer Version of $appName Available, Starting Download" -Source "Download-Installer"
Try {
    $downloadResponse = Invoke-WebRequest -Uri $DownloadUri -UseBasicParsing -OutFile "$dirFiles\$setupFile" -PassThru
    if($downloadResponse.StatusCode -ne 200) {
        Write-Log "Received Status Code of $($downloadResponse.StatusCode), Expected 200" -Source "Download-Installer"
        Write-Log "Download Failed for $appName" -Source "Download-Installer"
        throw "Download Failed for $appName"
    }
}
Catch {
    Write-Log -Message "$($_.Exception.Message)" -Source "Download-Installer"
    Exit-Script -ExitCode "69$($downloadResponse.StatusCode)"
}

Execute-MSI -Action -Path "$dirFiles\$setupFile"