you know when you're explaing a series of points and you start them with firstly...secondly...thirdly... Well, I got to fifth and it felt wrong. I couldn't recall ever seeing 'fifthly' before. How long can you correctly keep adding 'ly' to a number?
In my experience it's usually listed as second, third, fourth, etc, but when a list gets this long isn't it better to use bullet points or number the items
as cathfromsaron says - you don't need to say firstly, fifthly or anything; first, second... is fine - and in fact grammar books prefer it. (Think of it as being short for "The first point is...") Fifthly is bad enough, but wait till you try saying eleventhly and twelfthly.
Firstly, there is nothing wrong with 'firstly' - see Fowler.
The reason 'fifthly' may look wrong is that most people I hear do not pronounce 'fifth' correctly - they say 'fith', so they would say 'fithly'.
Also the same with 'sixth', which most people pronounce 'sikth'.
And 'eighth' which, despite its spelling, should be pronounced 'eight-th'. [I checked with a dictionary which gave the pronunciation as 'ātth'.]
Fowler, as mentioned above, has this to say. "Firstly, secondly...In such sequences where points or topics are enumerated the choice of form as between first and firstly is optional in modern English, as indeed it has been since at least the 17th century."
It could hardly be clearer...the choice is yours and there is nothing whatsoever wrong with fifthly. Lists very rarely get as far as eleventhly and if they were in danger of doing so, some better form of presentation would obviously be required.
Quizmonster is probably correct in saying that lists rarely get as far as eleventhly. There is at least one famous list that gets as far as 'twelfth' - 'On the twelfth day of Christmas, my true love gave to me ...' This (twelfth) is another word that is usually mispronounced as 'twelth'.