User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.5) Gecko/20041107 Firefox/1.0 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.5) Gecko/20041107 Firefox/1.0 Performing this action could result in possible system instability, if not, firefox will probably just crash if it doesn't eat all your resources first. In a nutshell: Firefox will attempt to keep opening up the file in firefox once assigned an "unknown" filetype (in this case ".dtd") when it should probably be otherwise treating it as a plain text file (in which this case, this is). Reproducible: Always Steps to Reproduce: 1. visit http://www.w3.org/TR/2001/REC-xhtml11-20010531/DTD/xhtml11.dtd 2. instead of selecting "save as", navigate to your firefox installation and set the ".dtd" filetype to open with the firefox.exe (unlikely that any other program is using this extension, if it is, find a similar example or temporarily disassociate the filetype) 3. click "ok" and let firefox open the file with firefox. Actual Results: Firefox repeatedly attempted to open the document with itself, spawning an endless loop of tabs or new instances of itself in the process. Expected Results: defaulted to open as a plain text type of file and only launched one instance. Works under any theme with any or no components installed. While I have had this continuously open just new tabs, I think if you don't have settings to "open in new tab", this will probably open dozens if not hundreds of new windows. Good luck catching the windows in time to stop the loop. While I'm not completely certain if it's the formatting of the document or if it's firefox's inability to detect the formatting of the file. Eventually, if this keeps up, it's going to bring the system it's running under to it's knees and eventually bury it :/ I'm really afraid to turn tabs off now (and with good reason).

addendum : also while that .dtd file asked to be "saved as" this one : http://www.w3.org/TR/2000/REC-xhtml1-20000126/DTD/xhtml1-frameset.dtd opened in firefox just fine without issue. So I am just guessing that it might be document formatting.

*** This bug has been marked as a duplicate of 167320 ***