Links opened by Thunderbird (or other application) will go to the Default Browser (using the Default Profile for Firefox as 'shown' in the profiles.ini file) as set in Windows "Default Programs" in the Control Panel. Location of that setting may be in a different in Win10 - I am using Win7.

But there seems to be a problem in Windows 10 with setting the Default Browser to Firefox for some users; many threads in this forum about Firefox not being 'offered' for selection as the Default Browser. With two Firefox installations you should look to see if both are 'offered' as a choice to be selected from.

In my profile.ini file:

[General] StartWithLastProfile=1 # I changed this to 0. Now they are opening in current windows with a new tab.

Content of profiles.ini has nothing to do with the default browser as that is an OS setting. That only affects what profile a specific Firefox version will use. Best is to make sure that the profile that is meant for the default browser (either 32 bit or 64 bit Firefox) is set as Default=1 in the profiles.ini and leave StartWithLastProfile=1, so that profile will be used.

Which browser is currently set as the default browser if you check that in the Windows Control Panel?

• https://support.mozilla.org/kb/how-make-web-links-open-firefox-default

You can check the setting of the browser.link.open_newwindow.* prefs on the about:config page. You can override how external links are opened via the browser.link.open_newwindow.override.external pref. If this pref has the default value -1 then browser.link.open_newwindow is used. The meaning of the value of the browser.link.open_newwindow.override.external is the same as of browser.link.open_newwindow

• 1:current tab; 2:new window; 3:new tab;
• http://kb.mozillazine.org/browser.link.open_newwindow

What it does changing the value to 0 is that next time you start Firefox, it will ask you what your default profile should be. Otherwise it will use the last created profile. So when you are using one profile, have another created, have the window open with the first profile, when you click on a link, Firefox will use the second profile. Since you have the first profile opened, Firefox can't use the same window and open a new tab, as it's told to use another profile, that means it have to open a new window, to open that link with the last profile.

Sorry, my initial answer may have made too many assumptions, or may have provided too much information and not enough of an explanation.

As far as identifying which version is running, about:support (in the URL bar) under Application Basics > User Agent you'll see WOW64; for the 32-bit version being run on a 64-bit Win OS and Win64; x64; for the 64-bit version.

Let's verify what you have going on and "fix" the Profile Manager appearing all the time. IMO, you shouldn't have to do that all the time, assuming Win10 knows you have both 32-bit and 64-bit versions of Firefox installed.

Please take this step-by-step.

1st: Does WIn10 - Default Apps > web browsers show either or both Firefox installations? If both, make sure the one that you want (64-bit you said) to be used for opening links from external programs like Thunderbird is selected. And while you're in Default Apps, verify Thunderbird is set as the default for Email - if that is what you want.

If only one appears, is it the Win64 version that you want as the "Default"? And if not, what was the order of installation of the one you want as the "Default"? I have no idea what Win10 does when two versions of the same browser are installed, I don't have access to a Win10 PC; I recently "upgraded" from WinXP to Win7 when I built a new PC last spring. And as I mentioned initially some users have reported an issue with Firefox not appearing in the Default Apps > web browsers in WIn10.

2nd: I am not a 'fan' of users editing the profiles.ini file and it isn't something for less than advanced users to do per Mozilla; there are other ways of accomplishing a "fix" without editing it and getting unexpected results, or creating a 'glitch' with how it is intended to work. "Been there and done that" far more often that I care to admit over the years, in learning first hand how the PM works. And IMO having to use the Profile Manager all the time is not necessary.

Restore StartWithLastProfile=0 to StartWithLastProfile=1 to get the Profile Manager working again. And to get the Profile that you want to use with the "Default Browser" the Default=1 line needs to be at the end of that Profile to make that the Profile that Firefox will Default to. Making sure that you don't upset the formatting of that file. i.e., line spacing is correct and no extra character spaces added.

3rd: And my recommendation for Launching the 32-bit version is to use Command Line parameters in the Target line of the Desktop shortcut, such as this to open the "other" version with the correct Profile. That will avoid needing to select the correct Profile from the Profile Manager. Example: "C:\Program-Files\Mozilla\Firefox\firefox.exe" -p default

"C:\Program-Files\Mozilla\Firefox\firefox.exe" is the "path to" executable for the 32-bit version as it would appear in the Target line unedited.

By adding -p default (with a space added after the .exe AND a space between the "-p" and the "default") you're telling Firefox to use the default named Profile. But you need to use the correct "name" for that Profile as it appears in the profiles.ini file by Name= . And if you put a space in the Profile name when you created that Profile it needs to be enclosed with quotes. AS IN "what ever" for a Profile name. "C:\Program-Files\Mozilla\Firefox\firefox.exe" -p "what ever"

Now that you are overwhelmed with information that may take some time to digest, here comes the bad news ...

Firefox isn't made to have two separate versions installed (due to them both using the same name for the "executable" file - firefox.exe) and when one is set as the Default Browser this issue may "rear its ugly head". The "Default Browser" thing won't work correctly when a different versions' firefox.exe process is already running. There's a "lock" file for the running process that won't let a another process by the same name be opened. IOW, if 32-bit is running and the 64-bit is the Default Browser, the 64-bit version probably won't open when "called" from an external program. As I recall from a real long time ago, a new window of the already running process will be opened instead of the Default Browser, as the user would expect to happen. But I haven't used multiple versions installed and multiple Profiles in the Mozilla endorsed "method' in real long time, so I don't know how it is intended to or actually works now. Too much of a limitation for me. (In past years that limitation was overcome by some advanced users by re-naming the executable file, but I don't know if that still works as I stopped doing that 10 or 11 years ago. I have my own method of getting around that limitation that I have been using for many years. A variety of methods is posted over here and in four earlier postings that are hyperlinked at the bottom of that posting. It is very overwhelming for the average user and not recommended by Mozilla, but it works for many Firefox users. It was started for the purposes of testing pre-release Builds while continuing to use the stable Release build. So it might be too intense for your 64-bit plus 32-bit installations alone.)

I'll try to keep an eye on this thread for any questions you may have and provide answers if you can handle the depth of this topic.

Ed