In what way does this incompatibility manifest itself?

Does it make a difference which program is started as the first?

Make sure to update both programs to the latest version and possibly do a clean reinstall to ensure that all DLL files are updated properly.

All the latest versions of the beta program, when I installed Thunderbird removed firefox , when installed firefox Thunderbird removed.

Do you mean that the Firefox program folder is no longer present when you install Thunderbird?

Firefox is usually installed here:

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

Installing another program shouldn't have effect on other installed software unless you would install all software in the same program folder.

I solved the problem, firefox no install in the same folder with thunderbird .