The /f/ sound is spelled with <f> or <ff>. The /ð/ sound is spelled <th> throughout.
The contrast is between a voiceless labio-dental and a voiced dental and alveolar fricative. The sounds are fairly similar but the /ð/ sound occurs in so few words other than function words that there is little confusion. I have left out the pairs in which a word contrasts with a contracted form such as fade/they'd or fats/that's, since they are most unlikely to cause problems.
The mean density value is 1%. The list makes 29 semantic distinctions, a loading of 63%.
brief breathe briefing breathing briefed breathed briefs breathes briefer breather differ dither differed dithered differing dithering differs dithers fair their fairs theirs fair there fare their fares theirs fare there fat that fee thee fem them fen then fence thence fey they fie thy fine thine foe though foes those fuss thus gaffer gather gaffers gathers heifer heather heifers heathers life lithe loaf loathe loafing loathing loafs loathes loafed loathed reef wreathe reefing wreathing reefs wreathes reefed wreathed rife writhe sheaf sheathe sheafs sheathes Suffolk Southwark whiff with
John Higgins, Shaftesbury, October 2010.