Is there a tool like Furigana that I can use on iOS Safari?
You can use a bookmarklet. Just sync your Safari or Firefox bookmarks with iTunes. Every bookmarklet you have on the desktop will be available on mobile Safari on the iPhone.

Or you can add the bookmarklet directly from within mobile Safari on the iPhone:

Step 1: Select the line bellow and copy it:

javascript:void(document.location='http://trans.hiragana.jp/ruby/'+escape(document.location))


Step 2: Navigate to hiragana.jp and tap on the '+' button to save a bookmark

Step 3: tap on the bookmarks icon then on 'Edit'

Step 4: Select the bookmark you just added

Step 5: tap on the address, then on the gray 'x' to clear the field

Step 6: tap that field again and select 'Paste'

Step 7: you're done