Have you looked here yet?

https://support.mozilla.org/en-US/kb/where-find-and-manage-downloaded-files-firefox?redirectslug=Downloads+window&redirectlocale=en-US

This is not an answer. I have already set my preferred download folder in the general settings as I said. Also, the link is about the download panel, it simply lists all present and past downloads, it doesn't say why Firefox sometimes is not respecting my settings.

You can't set the root directory on a drive as the default download location, Firefox wants to use a dedicated directory on a drive. This is likely to prevent data loss as clearing download data might cleanup the entire drive.

As a side note, if you prefer to read PDFs in a tab in Firefox, you can bypass the confusing situation that some PDFs are treated as web content and cached while other PDFs are silently saved to a regular folder by making a settings change. There are two options:

(1) Use the web content cache for all PDFs (override "attachment" disposition to "inline")

When your handling action is "Open in Firefox", all PDFs can now be opened as web content and saved in the cache instead of a regular folder. Here's how you set this up:

(a) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. I'm using this so I feel comfortable mentioning it.

(b) In the search box in the page, type or paste browser.download.open_pdf_attachments_inline and pause while the list is filtered

(c) Double-click the preference to switch the value from false to true

(2) For downloads that websites push as forced saves, present a prompt to ask you what you want to do with the file (similar to Firefox 97 and earlier)

(a) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(b) In the search box in the page, type or paste browser.download.force_save_internally_handled_attachments and pause while the list is filtered

(c) Double-click the preference to switch the value from false to true

I don't want to sound harsh but you keep avoiding the point. First, what you said is not correct. I have always been using E:\ as my download dir, that is my work/data partition and I like to put stuff there and I am free to do what I prefer, and Firefox *always* respected it without problems. Rather, once it happened to me to find my System partition damaged and I had to reinstall Windows RIGHT BECAUSE FIREFOX DID NOT RESPECT MY SETTING AND CRASHED RESULTING IN CORRUPTED C:\ PARTITION ! Please stop lurking in my hidden settings because I just changed them for two reasons: 1. to avoid the ugly Win10 looking windows bar 2. to use E:\FirefoxCache for the cache (again, to prevent damaging my System partition, and this setting should have been moved to the regular settings !!!!)

In addition, I have also set the option to always ask where download, and I am therefore always presented with a file dialog, which opens by default on E:\ as it should be. But in some cases, as with the link I reported in my original post, the download starts silently to C:\Users\Name\Downloads, without even opening a file dialog. Do you want a vid ?? Please don't tell me this is not a bug.

Please stop lurking in my hidden settings

No one is looking in your settings.

In addition, I have also set the option to always ask where download, and I am therefore always presented with a file dialog, which opens by default on E:\ as it should be. But in some cases, as with the link I reported in my original post, the download starts silently to C:\Users\Name\Downloads, without even opening a file dialog. Do you want a vid ?? Please don't tell me this is not a bug.

I abbreviated my previous comment because the problem was with a PDF, but now I'll provide more background and context for this discussion.

As you know, you can set different actions for different content types, including Open, Save, or Always ask. (Manage file types and download actions in Firefox) In the Save and Always ask scenarios, the setting to ask you where to save is applied. However, in the Open scenario, it is not.

Using the Open in Firefox option for PDFs

Firefox can't display PDFs without downloading and saving them somewhere. You can tweak some hidden settings to determine where Firefox saves the file, whether that is the web content cache, the "Save files to:" download folder, or the Windows Temp folder. I don't know why you aren't seeing them in your "Save files to:" folder when it is set to your E:\ drive; cor-el may have had the answer to that.

Background: inline vs. attachment disposition

If web servers don't specify how Firefox should handle a PDF, or if they specify "inline" handling, then then Firefox loads the PDF as web content with its original URL in the address bar. The PDFs are saved with other cached web content, not in your download folder. This is good.

But web servers can try to force a download by setting Content-Disposition: attachment if they don't want browsers to show the files in a tab. Firefox changed what it does in this case:

Before Firefox 98: Firefox always showed a download dialog, even though you had already told Firefox what you wanted to do, even when you checked the box to always do this in the future. It was kind of infuriating.

Firefox 98+: Firefox downloads the file automatically and then opens it. Because these are saved to disk the URLs start with file:///. By default, they are saved in your "Save files to" folder on the Settings page.

Is that the change you are noticing, or is it something else, not related to Firefox automatically opening PDFs in a tab?

Do you want to use either of the workarounds I posted previously (always open inline, or prompt where to save)?

This report on Reddit sounds similar: https://old.reddit.com/r/firefox/comments/1hoaye5/custom_download_dir_sometimes_not_respected/

I searched in Bugzilla (https://bugzilla.mozilla.org/) but could not find anything that sounded relevant.

I find this very frustrating. I always want to be asked where to download. That's what I have in my settings. Downloads: selected 'Always ask where to download files'. It's still set.

Firefox updates and I suddenly it stops asking me where to download and dumps it in downloads. So now I have to go in and move the damn thing. Please fix.

Hi Brett, the first step is to review the list of actions per file type and change them to "Always ask" so the instructions in the list don't override your general preference. See: Manage file types and download actions in Firefox.

PDFs are a bit special, because you often want to view them in Firefox seamlessly without having to make a decision AND without having them saved to the Downloads folder. In that case, you can keep the default PDF handling on "Open in Firefox" and use the setting change under #1 in the following reply to force Firefox to save them in the web content cache -- this is the setting I use:

https://support.mozilla.org/en-US/questions/1481736#answer-1698223

If this thread doesn't have the settings you need to get Firefox to handle PDFs the way you want, let us know what you want Firefox to do with them.