The phrasing on this site (the A-Z quoted below) does suggest that Mirena may shrink fibroids - but I can't find any information.
The reference you give silverlady also bears out what it says in the A-Z and which I quoted below ie there is some evidence that transdermal rather than tablet HRT may promote fibroid growth. Unfortunately the reference given in the A-Z (2002) is one of those that's not freely available and I'm not prepared to pay for it! In the particular study - the one you linked to - unfortunately as well as the mode of delivery and type of oestrogen, the amount of progestogen differed too - so the two groups were not strictly comparable - but I presume later studies (that I can't find!) do agree with that finding re tablet HRT and fibroids.
It is not possible to say from an individual case that an outcome had a particular cause - only trials using a good population size of women are able to ascertain this.
There do seem to be several different types of fibroid and I mentioned on another thread I think they are sturctured differently because are formed from different parts of the uterine lining - so will have presumably have different hormone receptors and may react differently to oestrogen and progesterone, not to mention the synthetic ones? There is also the difference in action of progesterone and synthetic progestogens - but it all seems somewhat technical and without knowing how it's all structured (the tissues and cell layers), difficult to follow!
I did find this interesting article which did seem to indicate a role of progesterone in increasing fibroids
http://www.ncbi.nlm.nih.gov/pubmed/23303565 but there would need to be trials to show whether it actually happened in practice.
The UK site says use Angeliq with caution with history of fibroids in the womb - so I would tend to agree with silverlady on this one - although - it depends on the size of your fibroid. I presume it's big enough to cause worry and therefore you would be right to be cautious. I have one but it's tiny and the gynae said it was OK to carry on with HRT.
The best thing is I said before to discuss with your specialist and if it's just a GP ask to be referred if you can and especially if the fibroid is causing problems.
This is a bit of a thinking aloud ramble but hope some of it is helpful.
Hurdity x