If Roblox is unable to run or provides a prompt to download Roblox, even though it's already installed. To check, perform the following:

Mozilla Firefox

1. Click the menu button in the top right of your Firefox window
2. In the menu, click the Add-ons button
3. In the window that appears, click Plugins
4. Find "Roblox Launcher Plugin", and select Always Activate in its dropdown menu on the right
5. Close the Add-ons Manager tab and launch a Roblox Experience

Apple Safari

1. Open Safari and click the gear icon in the upper-right corner
2. Click on Preferences, then the Security tab
3. Under the Security tab, make sure JavaScript is enabled

Google Chrome

Note: As of version 42, Chrome no longer supports plugins. 

Microsoft Edge

Note: Microsoft Edge does not support plugins