Is there a keyboard shortcut for "save link as" in firefox or chrome?

You can restore the old Alt-click "Save Link As" functionality in Firefox:

Open a new tab in Firefox. In the Firefox address space, type:

about:config

Click the "I'll be careful, I promise" prompt. Type

browser.altClickSave

in the Search space. Double-click on the browser.altClickSave line to change the value from false to true. Close the about:config tab.

Now when you Alt-click on a valid download link it will open the Save As explorer screen. All you need to do is set the correct location and press Enter. Once you set the download location, if you are downloading a lot of files into the same location, all you need to do is press enter. This will greatly speed up your download process.

I did the above process in Windows 7. It should also work in Ubuntu.

The other answers are correct for Firefox.

For Chrome it's even easier:

Option+Click (Windows: Alt+Click) saves the link-target to the download folder.