Configure Thunderbird to send patch friendly

Recently, I have tried outlook and web gmail to send patches to OpenBSD, but neither of them is an ideal choice. For example, the original patch is:

Index: dmesg.c
===================================================================
RCS file: /cvs/src/sbin/dmesg/dmesg.c,v
retrieving revision 1.29
diff -u -p -r1.29 dmesg.c
--- dmesg.c     1 Sep 2017 07:31:45 -0000       1.29
+++ dmesg.c     4 Sep 2017 08:55:50 -0000
@@ -65,12 +65,12 @@ main(int argc, char *argv[])
        int ch, newl, skip, i;
        char *p;
        struct msgbuf cur;
-       char *memf, *nlistf, *bufdata = NULL;
+       char *memf = NULL, *nlistf = NULL, *bufdata = NULL;
        char *allocated = NULL;
        int startupmsgs = 0;
        char buf[5];

-       memf = nlistf = NULL;
+       memset(&cur, 0, sizeof(cur));
        while ((ch = getopt(argc, argv, "sM:N:")) != -1)
                switch(ch) {
                case 's':

The actual effect of sending patch using above tools:

Index: dmesg.c
===================================================================
RCS file: /cvs/src/sbin/dmesg/dmesg.c,v
retrieving revision 1.29
diff -u -p -r1.29 dmesg.c
--- dmesg.c 1 Sep 2017 07:31:45 -0000   1.29
+++ dmesg.c 4 Sep 2017 08:55:50 -0000
@@ -65,12 +65,12 @@ main(int argc, char *argv[])
    int ch, newl, skip, i;
    char *p;
    struct msgbuf cur;
-   char *memf, *nlistf, *bufdata = NULL;
+   char *memf = NULL, *nlistf = NULL, *bufdata = NULL;
    char *allocated = NULL;
    int startupmsgs = 0;
    char buf[5];

-   memf = nlistf = NULL;
+   memset(&cur, 0, sizeof(cur));
    while ((ch = getopt(argc, argv, "sM:N:")) != -1)
    switch(ch) {
    case 's':

We can see the spaces in the left part of code are lost. After referring Completely plain email and Thunderbird (GUI), I decide to useThunderbird as the email client, but need further customization:

(1) All sent mails are plain text: “Account Settings” -> “Composition & Addressing“, untick “Compose message in HTML format“:

Or before sending mail: “Options” -> “Delivery Format” -> “Plain Text Only“.

(2) “Tools” -> “Options” -> “Advanced” -> “Config Editor...“:

Set mailnews.send_plaintext_flowed to false:

Now the patch format is correct.