XiTi, which based its figures on a sample of 32.5 million Web site visits that took place on 8 January, said Finland has the highest proportion of Firefox users, followed by Slovenia and Germany. It found that the open-source browser is used by 38, 36 and 30 percent of users in these countries, respectively.
The United Kingdom has one of the lowest proportions of Firefox users in Europe, accounting for only 11 percent of Web site visits there. The 20-percent overall figure for Europe is an average calculated from the figures obtained for each European country, according to XiTi.
But XiTi's figures should probably be taken with a grain of salt, as Firefox usage tends to be highest over the weekend, according to Tristan Nitot, the president of Mozilla Europe.
"We should emphasise that these measures have been done on a Sunday, when Firefox's usage peaks. The Firefox browser is less used during the week, as enterprises are more conservative when it comes to using a newer browser," Nitot said in a blog posting that commented on figures released earlier by the Web metrics company.
Other Web metrics companies produce more conservative estimates of Firefox's market share. In November, OneStat.com reported that Firefox had achieved a global market share of 11.5 percent, though it found that only 4.9 percent of people were using it in the UK.
ZDNet UK's Ingrid Marson reported from London. For more coverage from ZDNet UK, click here.