Multithread Download Manager Optimized von Comss

This is a new fork of Multithreaded Download Manager by jingyu9575: https://addons.mozilla.org/en-US/firefox/addon/multithreaded-download-manager/
This fork focuses on stability, download integrity, and compatibility with modern Firefox builds.
It improves handling of large files, adds safer fallback behavior for problematic servers, reduces memory pressure during chunk processing, and introduces adaptive threading to prevent stalled or corrupted downloads.
It also improves resilience against range/content-length mismatches, adds smarter host-based single-thread fallbacks (including SourceForge-like hosts), and includes practical compatibility hardening for Firefox Nightly-related API edge cases.