Maybe the files are send with different MIME types by the server and Firefox uses the folder that was last used to save that MIME type.
Does the download Open with dialog mention that type?

no, same mime types. For example yesterday: I saved a couple of astronomical JPGs from the same source, and FF changed download folder several times to folder I had used over the day. Since when shall FF show this new behaviour? I am using FF over years from an early 1.x version, all the time with a "download to last folder" configuration - and FF decided never to choose the folder on mime types, even while downloading different files like jpg, gif, pdf, doc from the same source!

Nov.9, 2011: Reported behaviour (changing download folders for same file types) still in FF 8.0 production.

Nov. 22, 2011: This strange behaviour happens significantly less frequently - only twice within last 12 days.

So, I think it is SOLVED for the moment.

Firefox V12 still has the insidious behavior - randomly changes save as folder even after adding the two items to about:config

browser.download.lastDir.savePerSite;false

browser.download.lastDir;I:\Pics\Sorted\GrandCanyon

Example: go to a web site which has a gallery of say 20 pictures click on first thumbnail to view full size image select save as, enter folder name, save image hit back to return to gallery click on second thumbnail select save as - folder name is the same as previous - good, save image hit back to return to gallery click on third thumbnail select save as - folder name is something random, perhaps even from several days ago!

This is totally unacceptable.

I am using Windows 7, 64-bit with all updates on a very fast I5 cpu, 16GB ram, 6GB/s SATA drives -- system is high performance and works great except for Firefox 12

Since 6 months, while using and upgrading releases (from FF7 up to currently FF12.0, on Win7 prof 64) that behaviour "unexpectedly changes download folder" still shows up. Sometimes after the 2nd or 3rd file, sometimes not until the 10th ... numbers are changing. So, former ideas "mime types could be a reason" won't work, because - in my case - the mime types of the downloaded files (i.e. series of pics from same address, series of movies from same address of my internet video recorder save.tv) are always the same (proved!)

Result: there is still work to do - good luck!

It is possible that you have saved data in the past from the site.

Try to the clear the "Site Preferences" via Clear Recent History to remove already stored data.

• https://support.mozilla.org/kb/Clear+Recent+History

@khelmsch_de - thank you for the reply - you are having the exact same problem with V12.0 as I am. It is not a mime-type problem as it happens when I am saving only jpg files.

@cor-el - thanks also for the reply - I have already tried clearing "everything" with ALL boxes checked. Doesn't solve problem. After a while FF starts randomly changing the save as folder to some previous value, perhaps even from days in the past! Arrrgh!

Update: I've just spent the last five hours finding mis-placed files caused by defective Firefox V12 software! EXTREMELY FRUSTRATING! Once I select a save as folder, Firefox MUST keep using that folder and ONLY that folder until I change it. It must not matter what type of file I am saving, what URL it comes from or anything else. I told it to save to a specific folder and that's what it MUST do. I am the user. I know what I am doing. Firefox is a tool that MUST DO WHAT I TELL IT TO!

From perusing posts from others on various sites, one common thing is that people still having the problem with Firefox version 12 still randomly changing the save as folder are primarily running Windows 7 Pro, 64-bit. Could be nothing, but then could be a clue.

@cor-el my "clear site prefs"-option is activated, but this behaviour shows up anyhow.

I decided to try one more experiment with Ubuntu 12.04 and Firefox 12. At first it had the same problem but when checking the about:config file I noticed the browser.download.lastDir.savePerSite entry was a string set to false instead of a boolean set to false. When I changed it (not easy by the way - had to manually edit the prefs.js file in my home directory) the behavior is correct. Good news.

Now I went back to my Windows 7, 64-bit machine and lo and behold, it was also set to a string instead of a boolean. I fixed it and now this machine works properly as well. So, looks like cockpit error after all - mea culpa

Note that when you do a copy and paste from about:config to one of these posts, the string false looks exactly the same as a boolean false so none of you could see the error. Hopefully others having this problem will find their about:config file has a string value instead of a boolean value.

Thank you to everyone who helped with this issue - it appears solved for me.

You can reset a pref via the right-click context menu to the default (shows as an empty string) and close and restart Firefox to remove a pref that was created with the wrong type.

• http://kb.mozillazine.org/about:config

yaggle, did u get an answere to this. I can not believe anyone would build a browser that did change the save folder, but I know u are right because I have been looking for a solution since Firefox 3.5. and they still haven't fixed it.

@yaggle: 1001 thanks for your solution

   [browser.download.lastDir.savePerSite as boolean=false] 

which seems to bring up the awaited behaviour ... NO randomly changing download folders since two weeks!!

Great idea!

best wishes from Germany, Klaus

khelmsch_de, thanks that fixed the problem I was having.