Firefox Exetension or Alternative - Repost

WiderNet has a custom browser based on Firefox. We need 2 extensions or work arounds. First extension needs to allow for local file download and local file open/run. We have our original extension code not based on the webAPI for a starting point the local file open was from open download, which is not being converted to webAPI. We welcome alternative code to be put in the source code as well since we compile the browser on-site and security is NOT an issue as it is used offline only. Second extension or work around is to allow for multiple instances of the browser open, but on different proxies.