This was mainly to address unwanted accumulations of files in the Downloads folder, due to a change in FF a while back - where content being viewed 'In Browser' (such as pdfs) were no longer placed in the system temp directory AND no longer clearing themselves. Opening documents also behaves normally again.

The only thing I would add for this is to ensure that you ensure "Always ask you where to save files"  is enabled (checked) in;
'settings' -> 'Files and Applications' -> 'Downloads' section.

Since it usually remembers where you last saved its not inconvenient to have this on, plus it ensures you are aware of what is saving where.

Cheers!

Thanks to DelTomix, there is a way to revert to the previous behavior by altering these 5 files in about:config:

- browser.download.improvements_to_download_panel - set to false
- browser.download.useDownloadDir - set to false
- services.sync.prefs.sync.browser.download.useDownloadDir - set to false (I don't sync)
- browser.download.start_downloads_in_tmp_dir - set to true
- browser.download.alwaysOpenPanel - set to false

Additionally, you might also want to double check the FF Settings options. It seems that FF would prefer that you open those documents in your browser rather than in your local environment and that 'local' option is getting harder to find. Sneaky!

I'll punt to DelTomix to fill in additional details.