Make a third party download manager default
Is there a firefox addon or method that will either engage the download manager of your choice, or catch and direct the download to your preferred download, much like Flashgot used to do in pre-Quantum days?
I use Free Download Manager. I have two addons related to it, but they are not a perfect solution. FDM has an option to "intercept downloads in your browser", but it's a kinda clumsy process.