see

• When I go to file/import, the import button is not available, i.e., shadowed out and will not allow import of favorites from MS Explorer | Firefox Support Forum | Firefox Help
  https://support.mozilla.com/questions/809095
• Trouble importing IE favorites | Firefox Support Forum | Firefox Help
  https://support.mozilla.com/questions/763524

-> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working.

Make sure that you do not run Firefox in permanent Private Browsing mode.

• https://support.mozilla.com/kb/Private+Browsing

You enter Private Browsing mode if you select: Tools > Options > Privacy > History: Firefox will: "Never Remember History"

To see all History and Cookie settings, choose: Tools > Options > Privacy, choose the setting Firefox will: Use custom settings for history

• Deselect: [ ] "Always use private browsing mode"

If you have problems with importing the IE Favorites in Firefox then export the Favorites in IE to an HTML file and import that file in the Firefox Bookmarks Manager. That way you also preserve any sorting that you may have done in IE with the Favorites.

• Export the favorites in IE to an HTML file (bookmarks.html): File > Import and Export
• Import the HTML file in Firefox: Bookmarks > Show All Bookmarks > Import & Backup > Import HTML: From File

See also:

• http://kb.mozillazine.org/Import_bookmarks ("Import from another browser" and "Import from file")