Your code does work for me — it returns a list of <a>. If you want a list of hrefs not the element itself, add /@href:
hrefs = tree.xpath('//a[i/@class="foobar"]/@href')
You could also first find the <i>s, then use /parent::* (or simply /..) to get back to the <a>s.
hrefs = tree.xpath('//a/i[@class="foobar"]/../@href')
#                     ^                    ^  ^
#                     |                    |  obtain the 'href'
#                     |                    |
#                     |                    get the parent of the <i>
#                     |
#                     find all <i class="foobar"> contained in an <a>.
If all of these don't work, you may want to verify if the structure of the document is correct.
Note that XPath won't peek inside comments <!-- -->. If the <a> is indeed inside the comments <!-- -->, you need to manually extract the document out first.
hrefs = [href for comment in tree.xpath('//comment()') 
              # find all comments
              for href in lxml.html.fromstring(comment.text)
              # parse content of comment as a new HTML file
                              .xpath('//a[i/@class="foobar"]/@href')
                              # read those hrefs.
]