You are going to want to perform a clean re install of Firefox, first you should export your bookmarks if you have many.
In case the problem is caused by a corrupted Firefox software then you will need to uninstall and reinstall it again.
Press Windows key + R to open up a Run dialog box. Then, type “appwiz.cpl” and press Enter to open up the Programs and Features screen.
Inside Programs and Features, locate the Firefox installation. Then right click and choose uninstall.
After clicking Yes at the UAC Prompt, follow the on-screen instructions to uninstall the Firefox browser.
Reboot your computer before attempting to reinstall the latest version of Firefox.
Click here to download the latest version of Firefox
Open the installation executable and follow the on-screen prompts to reinstall Firefox.
Check if the Firefox right click issue is still present.
Let us know how it goes, best of luck.